I strongly suggest not purchasing Prototype 2. Yes, it was fun. Yes, it was marginally well-designed. But over-all, the first one was much, much better. I'll explain why, and you can decide for yourself if my issues are overriding factors for you:
Prototype 2 wins. I'll give it that. It's gorgeous. Poorly optimised, so you're gonna get a crap framerate, but it's gorgeous.
2. Control scheme
Prototype 1 takes the gold there. P1's controls are well-mapped by default and make sense once you become aquainted with them. P2 changes some core controls for no reason, and because of the way they're set up, rebinding them will not help.
3. Movement and parkour systems
P1 wins, but mostly because of the control setup. It just flows better, not only because it's been designed better, but also because the controls don't appear to have been designed for a race of crab people.
P1 and P2 are actually somewhat tied gameplay-wise. P1's open-world is far more enjoyable and feels more "alive." Things do NOT despawn. I don't mean like they don't despawn for a long time, I mean they DO NOT. I once, to test it, hijacked a helicopter and left it on a roof down on the south end of the map. I then spent an hour or so screwing around in the north end of the map. I then returned and found my chopper in the exact state I had left it, ready for me to board.
NPCs react semi-realistically. Civillians will run in fear and panic if they see you changing state, military personell can be fooled enjoyably with shape-shifting, and overall the NPC life feels "alive."
P2 wins in events and missions. P1 ultimately has the story missions and that's it. There are side-missions but they're boring as hell and are repetitive, with no real incentive to play them. Granted, it makes up for it by having an enjoyable open-world to explore, but it fails to encourage playing the events. P2, however, has better mission design, even if it does get a little repetitive, and incentive is given in the form of power upgrades to play through the side-missions.
Things despawn BADLY. I found two NPCs sitting on some chairs and rotated the camera. When it was turned back, they were completely different races and genders. Vehicles left lying around will disappear as soon as your field of view is broken by buildings.
NPCs have slightly better animations but don't react well. Fooling military personell feels like I'm playing hide-and-seek with a babysitter. "Where's the horrifying shapeshifting abominaattiooooonn? I can't fiiiiind hiiiim. Is he heeeere? How about in heeeere?" It just gets old quickly.
P1's story is far, far superior, and not just because P2 suffers from sequelitis. P1's story is about a man put in confusing circumstances finding out his horrifying past and learning to live with it while being targetted by a shadow mega-corporation that has accidentally caused the destruction of New York. P2's story is about a man finding out his just-for-heart-string-tugging family has been murdered going up against a Saturday morning cartoon villain. To make it worse, P2's main villain is the protagonist of P1, the man you've come to be attached to, and ultimately see as a hero, despite the horrors he's been forced to commit. It feels like a cheap twist placed in for no particular reason other than the writers needing a way to not make you the P1 protagonist again.
6. Overall experience
I enjoyed Prototype 2 but only in a very brief sense, about as long as the story lasted, and not for the story, for the missions. While both games offer new-game+ experiences, I just couldn't bring myself to do it with P2. It felt like funding had been cut after the missions had been basically designed, leaving a programmer to write the story and a modeller to design the open-world. Compared to Prototype 1, where I'm currently on new-game+ number four, I just can't recommend Prototype 2. It's fun for a day and that's the end of it, versus fun for as long as you're creative.