I went with Xbox, because Kinect is pretty badass. Its fun to play some games without any controls and lots of robots made in the past couple years have used it.
I have owned both since 2007 and I have to say that the Xbox is better in virtually every single way (and the PS3 is only superior in minor things that don't matter to me at all). As to SEN being free and LIVE costing money, there is a very clear reason for that: SEN is god awful for playing and communicating with friends online.