I would pick Playstation 3 because it has all the games I want to play. I also have a Wii but I rarely play that. I'm also a PC gamer so yeah.
From what I've been reading a lot of you don't really know what you're talking about so I'm just going to explain different aspects.
Graphics: Obviously, the Wii doesn't focus on this much since it aimed to be cheap and affordable. Despite what others may tell you the PS3's and Xbox 360's graphics are about the same. With multi-platform games it will look better on the "lead" console(the console it was developed on) since the Xbox's dev kit was cheaper than the PS3's, most games were made on the Xbox and ported to the PS3. Extremes would be how Fallout 3 looks far less sharp on the the PS3 than the 360 and how Final Fantasy XIII looks far sharper on the PS3 than the 360.
Online Service: Let's start with the Wii, shall we? You can send messages to other Wii owners or to different email addresses on this system. The Wii shop channel offers a bunch of other channels and games, including a "Virtual Console" where you can buy nintendo classics like Mario, Zelda, and Metroid. However, there is little to choose from, and there aren't anymore games being added to it. Also, many of those games get re-released every few years anyway. Playing games online is free of charge, however, for each game you have, you have a "friend code". To play with your friends, you need to exchange these friend codes. This code system is very complicated as you need to add different friend codes to different games if you want to play with your friends. It's terrible, needless to say. There is also no form of voice communication, except for "Wii Speak", except it can't be used with most of the games that you would actually play online.
Now for the PS3. This will seem waaay better than the Wii's online. You have one account for all the games you play, so it's easy to start up games with your friends and message eachother. Using the Playstation Eye, you can also video chat. Voice communication can be done using any BlueTooth headset. Playing online is also free of charge. The Playstation Network(PSN) has a number of games, movies, Tv shows, and add-on content for games. Sony has recently introduced a service called "Playstation Plus". For $50 a year or $20 something a month(I think?) you get access to betas, get free content, and discounted prices in the Playstation store. There is also an achievment system called "Trophies". It was implimented in late 2008 so early PS3 games don't have them unless the developer updated their game like with Uncharted. Easy challenges earn you a bronze trophy, harder ones earn you a silver trophy and the hardest are gold trophies. If you get all the trophies in the game, you earn a Platinum trophy.
Xbox 360 is said to have the best service, and you know, I think it might. There are two kinds of memberships for Xbox Live, silver and gold. With silver, you can talk with friends and buy content from the store, with gold membership, you can play games online and do everything the silver membership gives you. Sliver membership is free, gold membership is $50 a year. That's right, if you want to play games online with your friends, you need to give microsoft $50 a year, where as the other two cost nothing. The Xbox's online store is basically the same as the Playstations. With Xbox, however, they give you a headset with the system, which the other two don't. Xbox also has some neat features like cross game chat, so you can talk to friends even if you're not all playing the same game or in the same game lobby/server. You can also send recorded voice messages. It's also easiest to use.
Controls: We'll start with the Wii again, okay? So, I'm sure you all remember how everyone was talking about the Wii and it's super cool motion controls right? Do you also remember how basic they were or how they were hardly used? So yeah, got some good games but a lot of them hardly used the motion control and all the games that did were really gimmicky shovelware garbage. Skip to Wii motion plus, adds a new dimension so the control is more 1:1. Get ready for a REAL Zelda game! Skyward Sword!
Playstation has the same one we're all familiar with. No change from the original PS1 controller or PS2 controller. Why change what works well? This time around it has six axis, you can tilt and shake the controller for different effects in games. Playstation is also going to release its own form of motion control. Despite what people may say, they have been working on motion control for a long time. They're not copying the Wii. It's called "Move" and it works by using the Playstation Eye which tracks a round orb on a controller that changes colors based on the color and lighting of the room. It gives the same 1:1 motion Wii motion plus gives if it won't be better. A game to look forward to would be Sorcery.
Xbox 360: People have said that it's got the most comfortable controller ever made. I call fandumb on that statement. Yeah, it works fine, it's great. Different from the Playstation, but works the same minus six axis. Yep, Xbox is also jumping on the motion control band wagon. You may have heard about Kinect. A controller free form of motion control that also uses voice commands. Too bad at E3 all they showed were a bunch of gimmicky Wii rip offs that I literally almost fell asleep through I'm not even joking.
Games: This is the most important part of choosing which console you want. The Wii has all the Nintendo games you would expect, for the most part. These include Super Mario Galaxy 1 & 2, Metroid Prime 3, The Legend of Zelda Twilight Princess, Mario Kart Wii, Super Smash Brothers Brawl, and Animal Crossing. There are also some third party games like No More Heroes, Mad World, and Red Steel. Some upcoming games to look forward to are Epic Mickey, Donkey Kong Country Returns, Kirby's Epic Yarn, and Goldeneye.
The Playstation has flagship franchises like Metal Gear Solid 4, Uncharted 1 & 2, Rachet and Clank, God of War III, Resistance, Killzone, and LittleBigPlanet. Upcoming games include LittleBigPlanet2, Killzone 3, Sorcery, another Rachet and Clank, Heroes on the Move, and Infamous 2.
Xbox 360 has Halo, Gears of War, Fable, Banjo Kazooie, Alan Wake, Dead Rising, Mass Effect and some other ones you can also get on PC also. Sorry to say that Xbox doesn't have as many exclusive games as the other two. Most of it's good games are multiplatform or also on the PC. Some of its exclusive games and frachises are also moving to PS3 as well. Upcoming games will be multiplatform aside from Halo Reach and Gears 3. Dead Space 2, Portal 2, Call of Duty: Black Ops, Metal Gear Solid: Rising, Dead Rising 2.
Sorry for the wall of text, just some things I really looked into when they first came out and I've been following it all.