I don't have a next-gen console yet either and have een going through the same argument. I've finally decided on a PS3 and here's why.
-- The only real console exclusive X-Box has is Halo... and to be honest, I like to play my FPS's with a mouse and keyboard. PS3 has MGS4, FF13, the inevitable God of War 3, and still other stuff that you can't get on an X-Box. IMO: Gears of War doesn't count as an exclusive if I can get it for PC.
-- Blu-Ray > HD-DVD
-- The only real upside I can see to the 360 is the XBLA releases, I would like some of the games they have on there, but that in itself doesn't give it the edge over the PS3 in my opinion.
-- PS3 still has online capability, just not as robust a network from what I hear...
Basically it all boils down to your play style, what kind of games you like, and what you're looking for in a console I guess...