I prefer xbox, the online lags much less because the money you pay for online buys better servers. Xbox also has chat parties. My xbox has a 120gb harddrive and I haven't ran out of space yet.
I suggest xbox over the ps3. I hear there are a lot of problems with the ps3 such as people hacking the network and all that. I would just avoid the whole thing, xbox is better.
If you want to play lots of cool exclusives, get a PS3. You won't have to pay for online play either, I never noticed any difference between the PS3 and the Xbox online play. Both got their lagg. People often say that you get less lagg on a Xbox but trust me, if your on a crappy internet connection, you get lagg, no matter if you play on a PS3 or an Xbox. Also, the PS3 got a Blu-Ray player, Xbox doens't so if you want to watch some good HD movies you should get the PS3. My opinion: Get a Playstation 3.