The 360 in my opinion is a far more superior and enjoyable console than the Wii and when Project Natal releases you'll have virtually the same experience the Wii offers but with much better graphics and games. I own all 3 current next gen consoles and of the 3 the Wii see's the least play and has collected quite a bit of dust.
The only downside for the xbox 360 currently is the hardware issues it has but other than that I don't see how you could go wrong with trading up like that except if the system is already damaged, like someone else said depending on where or who you're trading with I'd make sure it is atleast still under warranty and has warranty seal still and possibly check if it has been opened before.
Don't listen to these people man. No one here likes the Wii. The question is, does you or your brother like the Wii? If not, trade it, if you do, don't trade it.
The Wii is a great console when it comes to Wii Sports, Tiger Woods and such, but he will get so much more from an Xbox. If he can I would recomend having them both. We have both of them and the Wii is mainly used when we have our House Parties, but we keep Rock Band going on the Xbox as well. That is until one of our guests is brave enough to take me on in COD Split Screen, or NCAA 10.