The PS3's arent backwards compatible mate. The first models were, which is why they were effing expensive. But they got rid of it to save cost. They still havent figured a way to put it back in at a low cost but are planning to bring PS2 games out for download or an emulator in the future.
It all depends if your kids wanna upgrade or not as their PS2 will still be there to play on. If that puts you off, its best getting a 360 for cheaper.
BUT if you've got a HD tele and are a bit of a gadget man yourself, you'd be on it more than the kids. The Blu Ray and DVD upscaler on it is out and out quality. Will replace your DVD player.
As for the Wii.....erm...you'll play it all of a week and then it'll gather dust. Its good for novalty but its more of a toy than a gaming console.
Depends how old your lads are. The PS3 and Xbox are for people above 11 years old really as they're buggers technically to set up, install game data, saves, deleting, getting online, playing movies etc, and are expensive to throw about. The Wii is really aimed for kids and families.