As I've said in another thread, Prototype is a game of moments. It's memorable for the crazy fucked up things you'll do with your powers like jumping off the Empire State Building onto a civilian, using him as a human surfboard down the street, then jumping off him, switching powers in mid air and elbow dropping a tank.
On the other hand, you have inFamous. The electricity powers are fairly standard. There's the average bolt which is like an infinite ammo pistol, electricity bomb which are basically grenades, etc. It's a third person shooter in disguise as a superhero game. But it has interesting characters, a cool setting, and an intriguing plot. Whereas Prototype's plot is likely to get lost in a wave of blood and carnage, you'll remember inFamous for the experience as a whole, and you'll spend time discussing theplot, and where it will go next as opposed to any exceptionally cool stuff you did along the way.
Ultimately, I'd say to get both since they're both completely different representations of what a superhero game can be.
