I will say this, If you are truly interested in learning how things and why things work, Linux is the way to go. With the builtin programs packaged by the distros and the Terminal you can do everything. From what I gather, you want that kind of control and you want to learn a new system. I've been on linux a very small period of time but I thoroughly enjoy the level of customization and control. I currently have Linux Mint 17.2 but have already started looking into a new distro & DE to try. I've started learning Perl & Python as well, which is something I would have never done before without actually seeing a need for it.
I base my decisions on this:
New / Beginners : Debian Based Distros | Ubuntu - Mint - Debian etc
As for intermediate and Experienced users, You'll hear Fedora - Arch - Gentoo and others thrown around.
Enjoy !