To be honest I would go with LBP hands down if you don't have the money for both. Buy a copy of an older shooter like COD4 to tide you over (can get that for under 30 bucks) or something. I bet you anything LBP will be the game of record for the rest of 2008, I would hate to be left out of that coming train of awesomeness for another shooter ;).
PSN ID: ChosenOne feel free to add me