I'd much prefer to play a game like Brawl or Galaxy compared to Bioshock or Call of Duty 4. If it were my decision I would keep the Wii as I prefer Nintendo first party games to any other software on the market. As if Brawl would not be released here, MoH:H2 was all EA's fault not Nintendo's.
The optimum solution would be to keep your Wii and buy a 360 as well so you can continue to buy the best Nintendo games as well as the Xbox games.
Ultimately it's your decision, no one can make it for you.