The Wii is an excellent party game system and I'd recommend if you have company over often.
It is great from young and old. The included Wii Sports is absolutely great.
It has built in Wifi so as long as there is an open access point, it is on the internet.
The 360 is great for the hardcore gamer. It has great games RIGHT NOW and has the best online of all the systems.
On Xbox Live you can see whatever anyone on your friend's list is doing, you can then chat, they can join in a game (like GOW), you get game demos automatically, and extra content for games you already own.
Right now the PS3's online plan looks shit and I don't see anyone being better than Xbox Live. With what you get for Live, it is well worth the $50 a year.
That's less than $5 a month.