Firstly, inFamous, in which you play a blady-bonced chap called Cole (why are superheroes in games never called Dave or Ian?) who survives getting blown up at the start of the game and develops cool electrical powers and stuff. You can fire off bolts of electricity, shoot shock-waves, survive falls from the tallest buildings and climb virtually any structure in a massive open-world city. As the game progresses you get to hurl electrical grenades, shoot powerful electrical rockets, and even hover with your powers. There are plenty of missions and side-missions, and you can chose to behave like a goody-two-shoes or a complete wanker, which in turn grants you access to different powers (and changes the colour of your electricity from blue to red - very Star Wars). The game is driven by its story and some very nice cut-scenes, and although there are a few graphical issues (pop-in being the worst) the game looks very nice for the most part. I like like the fact that Cole is bald because I can identify with themes of (hair) loss. I just wish I could fry people with electricity as well.
On the other hand we have Prototype, which is basically a cross between Hulk: Ultimate Destruction, Spider-Man: Web of Shadows, Spawn and the Guyver. Oh, and a bunch of Resident Evil type stuff about a mutagenic virus. Basically you play as this total bad-arse called Alex Mercer (see, still no Dave) who wakes up on a slab in the morgue only to discover that he has super-strength and cool shape-shifting powers that allow him to transform parts of his body into weapons. He's also nigh-on invulnerable and can leap huge distances in a single bound (but not tall buildings) and use 'adaptive parkour' to free run over obstacles and up buildings in New York City. Best of all, Alex can literally consume people to replenish his strength! Yes, that's right, you get to absorb people into your own bio-mass, after which you can assume their identities, complete with access to their memories and abilities! As the game progresses you'll unlock even more powers, upgrades and moves, all of which will allow you to eviscerate people in a more efficient manner. And eviscerate you will, as Prototype is one hell of a gory game. Alex can slice, dice and even rip people in half with his powers! He also has the ability to hijack military vehicles (after killing the occupants in an unpleasant manner) such as APCs, tanks and choppers. As with inFamous there are a few graphical issues, but you'll be so busy killing things that you won't even notice. The story isn't as polished as inFamous either, but the way that it is revealed by consuming various people is novel.
Both games are pretty cool, but for different reasons. inFamous has the better story and characterisation, but Prototype is just sheer mindless fun that caters to your base emotions. Although Cole's powers are cool he wouldn't last five seconds against Mercer, who'd basically just eat him (literally). A lot of people are picking sides when it comes to these games, but I enjoyed them both immensely. inFamous probably has more longevity because once you've played though in either good or evil mode, you can go back and play the other way. Prototype is more linear and I don't think it's the sort of game you'd play through more than once, although I can imagine returning to it for short bouts of violence more often than inFamous.
It's a tough call between these if you are strapped for cash. If you're after a compelling story, more varied enemies and missions, better presentation and a bald superhero, go for inFamous. However, if you like mindless action and gratuitous violence (although I'm not sure video-game violence can ever be gratuitous), Prototype is the way to go.
