Trying a new game isn't a bad idea. Everyone seems to love P4G, so if you are into that sort of thing, i'm sure its worth a shot.
If you are totally in love with the franchise, then you should go with it. I mean, I still have the original Metal Gear PSone, MGS2 and MGS3 as well as the HD collection for both PS3 and Vita. If Disgaea is your game, then it would be sweet to be able to play all of them on your vita.