It depends on a number of different factors: firstly, your age. If you're in your 20's or later, then you may feel somewhat apprehensive about playing the Wii. You may feel more inclined towards the 360. Although, having said that, the Wii does look sleek and well-designed, with a number of adult-orientated games planned. Also, the price: the Wii is significantly cheaper than the other two consoles. If you're into graphics, then go for the 360 or PS3, when it comes out. But if you value gameplay, then go for the Wii.
You may have trouble getting hold of one, as everywhere has sold out of their pre-orders.