On the XBL Arcade you've got old-school titles like Double Dragon, Gauntlet, Golden Axe, as well as games like Worms and Castle Crashers that are all great multiplayer fun.
I'd probably get a PS3 if it was just for single player games and RPGs that I wanted to play on my own time, but realistically, I'm probably not going to be doing much of that as much as I'd like.
There's not going to be a Wii hardware upgrade right around the corner that I'd be screwing myself on if I bought one now, is there?
If the GameCube was any indication, there is unlikely to be any hardware update that would affect consumer choices for Wii "models" like we have seen with the PS3 or 360.
Mario Kart and Wii Sports are the only two games I can get my not-that-into-games friends to play. Wii Sports' bowling+beer is probably their favorite from Wii Sports, but that's definitely second place to Mario Kart. With Mario Kart, (both Wii and Double Dash) I've got them to let go for some extended sessions sometimes, which always makes me happy. I like it when I can get them to play for a long time, simply because the competition is fun. They've became pretty good at Mario Kart from playing it pretty consistently. If only they would realize they could do the same in other games.. but oh well. We've had some 4-5 hour Double Dash sessions after the bars, and.. that's my ideal night. :D I will recommend that you unlock everything in both Mario Karts in your free time, if you do end up buying a Wii+MK+remotes. That way it's ready to go next time you have people over to play. I found my friends enjoyed playing with their Miis quite a bit once I had them unlocked, and everyone loves Toad/Toadette in Double Dash.
Not since system-linked Halo:CE in high school has a game been able to get my friends to play a significant amount of time. We had 12-14 hour sessions on weekends back then... *tear of happiness* But now, they're all so fickle and unwilling to learn a game; they'll only play EA Sports games when I'm not around to influence their decision. I tried to get a friend to co-op in Gears with me about a week ago, and he gave up less than 10 minutes into it. "You have Madden?" *sigh*