Well, I have both as do a lot of people here, They both offer literally the same things, just different games, Your going to be paying for XBL if you get the xbox, but if you get the PS3 your going to be paying a lot more anyway, so it doesnt really matter.
As for online, I find XBL to be more fun, and i think it has games that are better online than the PS3. Although the Xbox hasnt got any massive names coming out anytime soon, but im sure Forza 3 will be in the planning if not announced and has a release date by the Time GT5 gets anywhere near the shelves, Plus, as has been mentioned, TDU is on The xbox.
If i was you id get an Xbox, since not as many people in the TDU community play on the xbox etc
It will all come down to personal choice in the end, Im happy its you stuck in the position and not me (im really indecisive)