All three cost about $200 to upgrade, and all three offer improvements:
Live 8gets functionality improvements in many ways, and the option is there for roughly $140 more to get the Suite, which includes Session Drums (looks really good) and a ton of other synths and sample libraries. Also, Live 8 paves the way for Max For Live, which should be just insane, as most things in Live can be modified with a Max patch.

Logic 9 Has Mainstage 2, which is a great compliment to Live in many ways, but Kore 2 does a lot of what I would use it for. Logic is my favorite workflow, but that's in part because I spent months years ago working out a workflow in Logic, and many of the things I learned ther could be used in...
Digital Performer 6 The odd duck of the bunch. Most of the target audience isn't doing what I would do with it, so things like glitches in the audio while tweaking the sequence don't bother them, but in many ways DP is the most set up for playing a single instance as a long set. It's pretty easy to create a template where every Chunk is a separate song in your set, and to rapidly rearrange that set. Sure you can do similar things in Live and Logic, but they require an even more convoluted process to make sure everything is kosher etc. (track automation could be a real mess for one)
So, the question is, what update is the most valuable in your opinion? I'm tempted to stick with throwing money at Ableton Live, but I'm also tempted by Logics workflow, and DPs performance design. (plus DP is the current CPU champ apparently)