Logic
I left ableton 6 for Logic 8 Pro and have yet to look back. Ableton was buggy as all hell then, however it has improved slightly.
Logic is an Apple product so naturally it is going to interface well on a native machine. Think about it...
The internal effects, instruments and etc. are worth the price of admission alone. Logic has some of the best filters and delays imo and that's just scratching the surface. Workflow is a breeze as they've done away with the "enviroment" although you still can access it should you wanna go deep.
I totally prefer Logic to Ableton but I'll give Live some credit. Most notably what they're doing with Max.
http://www.ableton.com/maxforlive maybe you've heard of it? It's an interesting bit of kit but one can't rewire out to Ableton and expect to use any of those cool/somewhat cool Max instruments, effects, and midi utilities.
Also, purely my opinion here: Live's synths, drumracks, etc blow if you wanna side by side em with Logic.
Take the above with a grain of salt though, as I'm not a dj or a remixer and couldn't care less about sample or time stretching, which I guess is one of the things Live is famous for.
edit
Don't be put off by Logic's steep learning curve as was mentioned a post or so above me. They've mellowed it out quite a bit since 7 (extremely steep curve, that one)
edit 2
Logic also has a chord memory function, bad ass imo