This seems to be a recurring question on the forum, so maybe sharing a few thoughts on this could be useful to someone.
Pretty much any GNU/Linux distro can be as powerful as you want, it just depends on what packages you install and what kernel you update to.
You can run Ubuntu with other DE's than Unity. The looks or the destop environment are not a good criterion to select a GNU/Linux distro, as any desktop environment can be installed on pretty much any GNU/Linux distro in a matter of a few seconds to a few minutes. And that goes for pretty much any application, not only desktop managers. Choosing a desktop manager and the applications that are written for that DE is another thing altogether, and is purely a user preference thing. I myself have two desktop environments installed on my systems: Gnome Shell (Gnome 3.6.2) and XFCE (4.10), and at login, I select the session I want to use, but I only have one GNU/Linux install, Fedora 18. In the Fedora 18 repos, there is also Cinnamon, MATE, OpenBox, LXDE, KDE/KDE Plasma, Enlightenment, etc... and if I want, I can fire up yumex and get any of them immediately and for free.
Thus the choice of distro is not a matter of how it looks/workflow/desktop environment. Basically the choice of GNU/Linux distro is purely practical:
- what works out of the box on my hardware?
There are 3 kinds of distros: 1. conservative distros that use an older kernel and are focused towards providing maximum functionality and speed on older systems, like Vector Linux which is Slackware based, or Puppy Linux which has two variants, Wary and Racy, whereby you can select the one that corresponds to the age and performance of your system; 2. mainstream distros that use a modern kernel that is 2 to 5 versions old, like Debian and whatever is based upon it, i.a. Ubuntu and it's derivatives, which are the most popular non-bleeding edge DEB-based distros, or like Open SUSE/Rosa/Mageia, which are the most popular mainstream non-bleeding edge RPM-based distros. This type of distros often focuses on Userspace and software compatibility (do you want your gaming USB headphones with surround sound to work with it for instance, go with such a distro); 3. cutting and bleeding edge distros, that use a very modern kernel that is maximum 1 generation old, whereby bleeding edge distros use the most modern kernel. Basically, cutting edge rolling release distros like Arch and its derivatives (i.a. Manjaro Linux), are for power users, because they roll out the latest features as soon as they can through updates, and sometimes that causes serious problems on your machine, and you have to solve those problems so you have to know your way around. Manjaro Linux uses its own repos so that they can filter out the poisonous updates somewhat, which makes Manjaro a user-friendly and efficient way to use Arch with all the benefits of the speed of Arch and the AUR, but without most of the poisonous packages updates. Non-rolling release bleeding edge distros like Fedora (which is basically the bleeding edge testbed for RHEL/CentOS) are the frontline of GNU/Linux, basically most of the linux kernel developers are found in the ranks of the industry giants that use and make the Fedora/RHEL/CentOS distro, like RedHat itself, Intel, IBM, the Linux Foundation, Gnome project, etc... that means that Fedora/RHEL/CentOS is not focused on detailed application software compatibility as Canonical (the Commercial entity that makes Ubuntu, which is also a relatively important contributor to kernel development by the way), but more on hardware compatibility, security and performance. So do you want your latest and greatest hardware to work out of the box, go with Fedora. Fedora has the least problems with drivers and hardware of all GNU/Linux distros, as long as the hardware is less than 6-7 years old (there are exceptions, like HP laser printers for instance, which always work in linux regardless of their age, or on the other side of the medal, Canon scanners and printers or Gigabyte high-end motherboards for instance, which always never work really well in GNU/Linux because the manufacturer doesn't provide any info to the kernel developers and doesn't provide any decent proprietary drivers either, they basically tell GNU/Linux users to buy Windows or fuck off.)
- what answers my security requirements?
There are three types of GNU/Linux distro families in terms of security: 1. The distros that use SUSE's SELinux system, like OpenSUSE, SUSE, RHEL, Fedora, CentOS, which is the most complete linux security system, that monitors and real-time protects the whole system and all processes, but can be annoying sometimes as it has a habit of popping up warning notifications regularly (like 1-2 per day). You can set it to permissive mode which means that it will only warn, or enforcing mode which means that it will demand explicit user intervention to make exceptions and will block all suspicious calls (which rarely reversily affects any aspect of the system or user experience by the way). Fedora also includes firewalld, the new firewall reference for GNU/Linux, which is a great improvement over anything else. 2. The GNU/Linux distros that use AppArmor, which is a profile-based application sandbox security type thing that Ubuntu uses. This requires some configuration, unlike SELinux. First of all, there are no profiles available for all apps, only for some, and then some really important profiles are disabled out of the box, so for instance you have manually enable the Firefox AppArmor profile in Ubuntu. But if you have it configured, it allows for a good degree of security and works well. 3. The DIY-distros: distros that configure nothing out of the box, but require full user configuration, like Arch, can be as secure as the user wants them to be, but the user has to install and configure everything manually.
- what degree of FOSS mentality has it got?
There are distros that are more FOSS-focused, like Fedora, which is very strict in it's Free and Open Source principle, whereby for instance once you install a proprietary driver, your kernel is tainted and you cannot get bug follow-up support. This is annoying for users that have tasted the benefits of the great Fedora support. Basically, Fedora has an automatic bug reporting tool, that (if you allow it, because they respect privacy at all cost, and it's opt-in, not opt-out) follows up the bug on your system without need for user intervention, that means: there is a system specific bug with some package on your system, the system detects it and files a report, then the development team will solve the bug and push it out to your system in an update, and that mostly takes a lot less than 24 hours, and your system was never down and you didn't have to do anything. Great right, well, install nVidia proprietary drivers and it's gone, because the proprietary drivers are not open source, so the developers cannot "look into them", so they can't solve anything. Fedora allows you to get the maximum benefit from FOSS, but it comes at a price, and that price is that not all hardware has open source drivers that perform the same as proprietary drivers.
Other distros like Ubuntu don't have a consistent FOSS or privacy policy, like they ask you to install the non-free repos if you want non-FOSS on your system (like MP3 support or some video formats), they make it easier to get proprietary drivers by using automatic jockey notifications, and they integrate clearly privacy invading features from commercial entities like the Amazon lens or the automatic commercial software availability in the Software Center.
Some other distros are very relaxed when it comes to FOSS policy, they simply prepackage the proprietary drivers and codecs.
- what do my customers/employers/university/... use?
This is also very important, it's obviously best to use the GNU/Linux distro your study/work/hangout/hack community uses.
- image?
There is also an image issue, as in "what is the archetypical ...linux-user look like" and all the social pressure that comes with that. But that's simply bullshit, and as always, the crowd pulls you back down in the foodchain-pyramid, e.g. if your skills are higher than those of your hacker friends or fellow students or even teachers/professors, and you keep getting convinced not to go outside the given path, you won't be using your skills to maximum extent. It's not because some dudes on some forum told you to use this or that or do this or that, that it will work for you with your skillset. Everyone needs to make their own decision, and nothing comes easy, if you want to find out what suits you, you'll have to study it and experience it, there is no fast and easy way.