You should ask yourself this question: Which do you like better? RPG's or creating games?
Because this is what it will boil down to.
ME2 will have high replayability if you're the type of person that likes playing RPGs over again...or want to see different story choices.
LBP2 will have high replayability if you're into creating worlds and games/mods. The possibilities are practically endless and if you research a little, there's hundreds of LBP2 mods out there that are just amazing.
My opinion would be LBP2, even though I absolutely LOVE ME2. I think the option to create/upload/play other LBP2 mods makes the game a far more bang for your buck than ME2.