I'm sure the Alcon kits are great--as noted above, they have some users here, and I know they're used in some Euro racing series.
I went with a StopTech ST60/ST40 setup and have been very happy thus far. None of the odd surface rusting issues on bolts like a couple of people have had on their Trophy kits (see recent thread in Brake/Suspension topic section). Fit and finish seem top-notch. Their performance has been fantastic. They have plenty of options for pad choices, from the very good StopTech Street Performance pads at around $200 for the whole car, to more exotic pads and everything in between. Their bridge design allows super-easy and quick pad changes. And they're the kit used by Turner Motorsports on their racing M3s, and also many other competitors in American road-racing series.