I don't have both, but quite honestly, I would recommend either a Wii or an Xbox 360 over a PS3. Here are my opinions.
PS3
Pros:
Face it: gorgeous graphics
Plays Blu-Ray DVDs
Online service is free
Hard drive
Cons:
over five hundred dollars
Online service is untested
only partially backwards compatible: Not all PS1 or PS2 games will play and many apparently look worse.
Blu-Ray may not end up being the standard, in which case you'd be left with the Beta of next-gen DVDs
Every game on the system is also on the 360 (excepting Resistance: Fall of Man).
Lots of launch problems (HD resolution, compatibility, breaking, etc)
Xbox360
Pros: Great game library - Gears of War, Halo 1 and 2 and soon 3, loads of others.
Hard drive
Did I mention Halo?
X-box Live is the STANDARD for online console gaming
Over a year old with strong 3rd party support and some big games from Microsoft publishers
400 for the hardcore gamer system (do NOT buy the core system!)
You can buy an HD-DVD drive for it
Online store to buy movies and tv shows
Cons:
Xbox live does cost money, though not a lot
Not all original Xbox games work on it, though most of the big ones
still a bit pricey, but there's an awful lot going on for that cost!
Wii
Pros:
Unique and super-fun motion-sensor controller that IMO works very well. There's a lot of untapped potential but two months in some games have taken great advantage of it.
Classic Nintendo franchises: new Zelda, Mario, Metroid, etc etc will not be on any other system
Plays every single Gamecube game without problem
"WiiConnect24" has lots of potential. Currently the virtual console store, weather forecast and Opera browser "channels" are online, with news channel and online gaming coming soon.
Virtual Console store has great retro-gaming. Value is based on what you buy. Some games aren't worth a dollar, some go on ebay for WAY more than 5-8 dollars Nintendo is asking. Plus Genesis and TG16 games!
The new Zelda is one of the best games of all time, period. Pretty much.
Definitely the multi-player system of choice
accessible to non-gamers
Cons:
Not as graphically powerful as other systems, though still has lots of juice under the hood.
Does not play DVDs. Up to you if that's a major issue or not.
question marks about hardcore gamer support...
Bumps in figuring out the best use of the Wii-Mote in some game genres - no real successful first person shooters yet
And of course the cons of both Wii and PS3: Trying to find them.
I'm a nintendo mark from way back. I skipped the GameCube... so for me, I not only can play all the brand-new games, but I can pick-up all the good Cube games for cheap. I want to play Metroid, Zelda, Mario, Mario Kart, etc etc. I'm looking forward to Smash Bros online! The Wii has a few issues yet (no online gaming at launch? Aww.) but I'm pretty stoked so far. I can't emphasis enough how amazing Zelda is.
I would not have likely bought a new system this year if it wasn't for the Wii. If I was to buy one of the other two, the 360 definitely has my vote. If you're more into a "hardcore" game system, I think the 360 is great value for the money and will go far.
My final conclusion: There will be no clear winner in the next generation. Sony's dominance is over thanks to the issues with the PS3 launch, Nintendo has captivated an entire new audience along with some hardcore gamers, and Xbox has gained a devoted following and has that year of lead-time which they've used very well.
It'll be a three-way deadlock, baby.