As far as the included base tunes go, they'll do about the same thing. Predator or Xcalibrator. Both will lean it out a bit, advance timing, calibrate for gears, shut off rear O2 sensors, etc. The difference in the end user tools (handheld tuners) is more in the features of the tool. What can it adjust? How much? What other capabilities do you get? Do you want to tweak the tune while in your car? Or do you want to tweak the tune on a desktop or laptop PC?
If you're looking at a custom tune made by a professional using dyno data, then it really lies in the skill and experience of the tuner. Not the brand of tools he/she uses.
As far as support goes, I've heard good and bad about both sides. If you read other message boards and hear all praises, check to see who sponsors that board. You'll be amazed at how much bias there is out there. Check out the message boards that cater to both brands. You'll get a much better comparison.
Also watch out for the brand-loyal people. They crack me up. They'll chant "Go brand X!" or "Brand X all the way!" without really backing up their cheers.
I have some of my personal comparison notes in my cardomain thing.