I don't see how you get 50$ off a trade in, as I think I was offered only like 25$ for my MW2.
Anyways, as for the given scenario, I recommend keeping Bioshock2. HGSS are essentially the same game with a few differences. Wild pokemon, and "Base" legendary. Bioshock 2 isn't as good as the first one, (IMO) but it will probably bring you more entertainment than spending double the money on something you can play twice by resetting.
As for the desired scenario, try getting that miraculous 50$, and put it towards a game that's worth spending money on. My list would be:
MW2
FF -Whatever number the new one is
MGS4 (Kinda old but so good)
Warhawk (A personal Favorite)
LBP
Wait for the new 2011 sports games
Uncharted 2- Drake's fortune
Assassin's Creed 2