cross edge is not for casual jrpg players. it's hard at start, one of the hardest, especially if you play hard mode. (dieing in a random battle?), it's not a hack and slash game, it's like valkyrie profile and spectra souls, needs strategy every battle. srpg game.
cross edge, will take you more than 40+ hours to finish, 100+ if you want to get platinum.

prototype, it's like GTA meets HULK.
prototype can be finished with in 10-12 hours. then it becomes a free roam sandbox game after you finish, just to do the insane trophy requirements.

i got both,
I'm biased, if i have to buy only 1 right now, I'll get cross edge, for me JRPG > sandbox game. jRPG's are becoming rare now. hope ffxiii will revive the genre.