Necroing an old topic from the depths of the dead.
Last week I bought Prototype off from Steam fairly cheap, I regret not getting it last time it was on discount. As for the choice between Prototype and inFamous, it was fairly easy! InFamous is not available for PC.
In any case, Prototype is a lot of fun despite its flaws. Unlike a lot of games, Prototype manages to make the player character feel like he's actually damned strong and fast. There are a lot of different moves you can pull off, the downside is that usually you stick and learn only a few of them because it's at least to me impossible to remember all of them. The world is filled with plenty of different missions, but as said before, they're mostly challenge type missions designed to suck up player's time. Doing them rewards you with experience points (or whatever they were called) and allows you to buy ability upgrades. So they're kind of essential.
But most importantly. The game is fun to play! Eagerly waiting for Prototype 2 which should come April next year.