Xcal2. Love it, easy to use, looks good (does this really matter?), and is the latest and greatest with future updates in the work. Soon, it'll be able to datalog and change parameters without having to hook it up to my laptop.
I took a 3 hour trip the other day, and datalogged all sorts of things from vehicle speed and MAF volts to watching the coolant temperature as thousands of love bugs clogged my radiator (which was fine, BTW).
Thanks for your input guys; I'm really leaning towards the Xcal2. I had a Predator in my Mach, and it worked great, but I've read alot about the Xcal2 and most of you seem to have had great luck with it, so that will probably be what I go with.
I have a Xcal1,so I would recomend a Xcal2. Also,I have a few reasons SCT is better than Diablo. But in all reality,it comes down to your local tuner. If the best tuner in town runs diablo then go with diablo,or if he runs SCT then get that.