I use the Personnal License from the website, I don't want to use the steam version, since I don't want to use steam, it's to much a pain IMO, the licensing conditions are the same If I remember:
-One person allowed to use it, all functionnalities included (don't take my words, just see the Compare features of this page, and read this: How licenses works )
The diff?rence? The steam one is tied to your steam account (aka: you need steam to be launched, and connected to the right account), also, if steam just don't work for X or Y reason, you can't use C2 since Steam can't launch.
EDIT: the thingy alerted me, sometimes it don't work well, but that's right, I got an alert ^^