It depends what you want it for. The PS3 has a Blu-Ray drive so can play them obviously, the 360 has a better collection of games at the moment though and they're cheaper.
Xbox Live costs �40 a year which IMO is well worth it. Charging for it means you cut out a lot of the annoying kids who just want to ruin games for everyone else and swear at you, although there still are a lot of them, but you get the same with any online gaming. The PS3 online side of things is free, but it nowhere near as comprehensive as Xbox Live and there are connection issues with it.
Personally I'd go for a 360, but don't buy the Arcade pack - the cheapest one. Plump for the Elite or the Premium, they both come with a hard drive that's essential for online gaming. The Elite pack has a larger hard drive and is around �260 and is more recommended especially as Xbox Live now has movies and TV shows available for download. Whilst the PS3 has the Blu-Ray player, you can buy considerably better Blu-Ray players for about the same price so you're not missing out
Hope that helps a bit