They're the same price, which is better?
I have the HAF XM it's a great case with plenty of room to work in. Never built in the C70
c70's cool, good room and everything, looks cleaner than the cooler master, to me.
but I always hated the side window, stupidass fan mounts. you'd need to mod that, cut out your own clear side window, there's a youtube vid of that somewhere
I can tell you a few things about the C70 because I built a system in it a while ago for a friend.
It has a lot of room (although you might want to get creative for a thicker 240mm radiators if you choose to go for a custom water loop, but something like H100i won't be a problem at all), great airflow, excellent cable management, large side window (can be rotated or swapped with a custom clear window very easily, whichever you like), easy removable dust filters and almost every part of the case is made of metal. However there is no rubber pads for the PSU mount. Some people might find this bad because the PSU could be scratched or create a rattling noise because of the vibration, but based on my last build, I didn't hear a rattling noise or anything and everything looked just fine.
For the HAF XM, I personally never built a system in this case, but if I'm not mistaken, the dust filter for the PSU is like on the floor of the case, not outside the case like C70, so it might be a bit of a problem if you want to clean it. Other than that, I guess it is as good as the C70.
I decided to get the HAF XM. The main reason is the extra room and I personally like the way it looks better.