At the moment Online wise it's around £35 a year for Xbox Live and it's free for the basic service on PS3 which includes the online games, PS3 is also better online in terms of arseholes, Xbox Live is full of foul mouthed xenophobic prepubescent American kids, if he plays against strangers in open games, don't use the headset, soon as you open your mouth and they hear an english accent you get a turret of abuse and if you give em lip back it just makes things worse, people have been complaining to Microsoft about it for years but it falls on death ears. no one says anything much on PSN mainly I think because you have to buy a Bluetooth headset for around £50+ to actually be able to understand what anyone is saying and even then it's still awful because the amount of data space they leave open for the voice chat is tiny so it all sounds rubbish.
Game wise tho, there are more games released on 360 then PS3, PS3 games are ports of 360 one, and as 360 is normally the lead platform, the PS3 versions tend to be inferior. As Sony are a Japanese centric they don't really go in much for online games in the same way the west does so many of the PS3 exclusive titles feature none or poor online features.
Also PS3 games tend to be more money then the 360 versions of the same title, they all so tend to drop in price less after release.
I play online with both and the 360 is the clear winner, more people, better games, better service, and I'm sure that over the course of a year the online subscription fee is canceled out by the over all lower cost of the games, so it all works out he same in the end.
Just be careful of the foul mouthed yanky kids, there are plenty of gaming forums and websites on the net where people pass around there gamertags, better to add a few of them to the friends list and play against them, then venture in to the open games.