Which system you should pick depends in part on how exacting and tactical you want to be with things. I haven't played GURPS myself, but with Hero System and M&M, these are essentially two sides of the same coin. Hero System is a far more rigorous rule-set, including things like movement rates measured in 1m hex squares, a large variety of martial arts maneuvers for melee and ranged, both armed and unarmed, and so on. All in all, it has a very tactical feel, and makes for a grittier game, which may be more appropriate for your game. M&M, on the other hand, is d20 based, so most players will already know 90% of the in-game mechanics, allowing you to focus on world-building. However, it is a more free-wheeling, comic book style experience. Since they were both originally designed for the superhero genre, a good parallel would be to say that Hero System is like Batman taking on a bunch of mooks in an abandoned warehouse, while Mutants and Masterminds is like pretty much like Spider-man and Green Goblin doing an aerial battle over New York. If you're looking for a sense of realism, Hero System is the better choice. If you want epic battles, go with M&M.
Now, both systems have multiple versions out. I've played Hero 4th, 5th, and 6th, and the core mechanics have remained largely the same, with minor changes, throughout, so whichever of those you can get will be fine. I've played M&M 1st, 2nd, and 3rd, and there are fairly substantial rules changes between them. I would recommend going with M&M 2nd. If you can't find the books for either system through legit means, they are freely available through the internet piracy community.