Each company has pros and cons: and before anyone starts, I OWN all 3 of these systems now, so I know what I'm talking about. But this is just my opinion, so mileage my vary.
PROS:
Nintendo: Wii shop channel is a great addition, I've found a ton of classic games to purchase and the fact they are supporting the Genesis, Master System, Neo Geo, Super NES, and TG-16 makes it even better. Cost is reasonable, especially for a machine designed strictly for gaming.
Sony: FREE online gaming. Blu-Ray, pretty decent selection of games exclusive to it (MGS 4, Resistance:Fall of Man).
Microsoft: Gears of War, Halo, and Viva Pinata (both were released on PC as well) are both really good exclusive titles. Marketplace has a ton of cool stuff, if your willing to shell out cash for it. The Netflix add-on coming soon will be well worth it for some (I already use my 50" tv as a computer monitor, so I can stream netflix anyhow, but being able to do it from the xbox will be kinda neat.)
CONS:
Nintendo:The points system sucks, I've had 100 points leftover for freakin' ever. If you could buy them in alternate amounts other than 500, 1000, 2000, and 5000 I'd be set. Not a lot of online games outside of Mario Kart and Mario & Sonic @ the Olympics (I've only ever played Mario Kart online, went fairly smooth) My only other complaint is the total crap that is having the motion controls. Its super fun for games like Zelda, but Smackdown Vs. Raw 08 is freakin' terrible on the wii. They need to give you the option to use the classic controls for more games, or at least use the controller in a classic controller function like they did with Paper Mario for the wii. That game utilizes the controls perfectly!
Sony: Load times suck! Seriously, MGS 4 took 15 minutes before I could anything. First I had to download an update. Then when the game started, I had to watch Snake smoke for 10 minutes while it "installed" the game to the hard drive. Each act has to install. Similar problem with Bad Company and the new Star Wars game. Bad Company took about 20 minutes of waiting for updates to download and install.
Xbox: Paying 50 bucks a year for online gaming sucks, especially considering I only ever play Gears and occasionally Beatiful Katamari online. I tried playing the Bigs but no one else was online for it. Unless your on Gamefly or constantly buying new games, unless it was a huge hit like Gears is, good luck finding people to play online with. Again, points system to buy things is stupid. I understand they gotta make money somehow, but why do I need to first purchase points, I've got a credit card, take my money and give me my movie/ game. And why should I pay for wallpapers and icons? I'm already paying for the service, kinda seems wrong to me.