[gmookher]

Okay, so thanks to you all for the great information. Both are excellent brakes, I havent yet finalized which one will work best for my usage, but its pretty clear for track user the winner is the Essex Sprint kit's CP8350; and pretty clear for the DD guy who may track but also needs weather protection, the ST-40 is the way to go.

While doing my own research off this forum, I found the cut and paste below, from J Ritter of Essex off of a corvette forum:

I'm uniquely qualified to answer this question...I was the sales mgr. at StopTech from 2003-2008, and I now work for Essex (the importer and distributor for the components used in the AP T1 kit). I worked with the AP folks and Gary at Hardbar to spec the T1 kit. Here's the breakdown as I see it.

Calipers
Anti-knockback springs
AP Racing has them, StopTech doesn't. People severely underestimate how valuable these springs are. After going through S turns, you'll have a much lower pucker factor in the next brake zone with the springs installed.

Piston type
StopTech= cast aluminum
AP= machined stainless steel
Stainless is far superior at keeping heat out of your brake fluid

Weight
ST-40 weighs 7.94 lbs. without pads
AP Racing CP8350 weighs 4.8 lbs. without pads...roughly half of the StopTech's

Dust Boots
StopTech is a street caliper, and as such uses dust boots. CP8350 has none. If you've ever run dust boots on the track, you'll find that they burn up and cause a big mess almost instantly. Their only value is if you're driving on winter or debris/dirty roads. As soon as they go on track they essentially lose all value.

Seals
Both calipers have quality, high temp seals.

Hydraulic protection
ST-40 has none, while the CP8350 has it for both bleed screw and crossover tube. This feature protects the hydraulics from track debris, and when you're swapping wheels.

High quality forged aluminum
The caliper bodies on the ST-40 and CP8350 are both high quality forged aluminum parts.

Pad choice/cost/size
Both calipers have tons of pad options. The pad surface area on the ST-40 is slightly larger, but they are a few mm thinner than the pads used in the CP8350. Overall pad mass is very comparable, but the nod goes to the thicker pads in terms of wear rate. On average, pads for the AP caliper are $100 less per set for the same compound.

Stainless Steel hardware
Both calipers have good stainless steel hardware.

Footprint/size
In addition to weighing much less, the CP8350 has a substantially smaller footprint and offers superior wheel fitment.

Finish
StopTech= painted. Look pretty, but will not look as good after hard use. Paint and powdercoat tends to change color dramatically at track temps. If you're driving through road salt, they will be better protected than anodized.
AP Racing= anodized. Look 'racier' (like what you see on full-blown race cars like C6R) and do not color shift nearly as much.

Rebuild Service
Essex offers a lifetime professional reconditioning service. For a $125/caliper you can send your calipers to Essex and have them rebuilt by the same techs who service NASCAR and ALMS teams. StopTech can rebuild, but I'm not sure how formalized their procedures, policies, and prices are at this time.

Discs

Quality and Price
Both companies offer high quality discs for the kits in question, at very similar prices. The AP's are actually a bit cheaper for replacement iron ($249) vs. $275 (i think it's still this much) for the StopTech's. Discs from both manufacturers are quality castings that are crack resistant and will last a long time under heavy track use. Due to the enormous number of variables, it's tough to say one would give you more longevity than the other. There are plenty of users of both on this forum who are getting the type of wear rates you describe.

The fact that the AP's are able to get similar wear, even though they are over an inch smaller (12.8" vs. 14.0") is notable however. Their 70 vane internal construction, and patented J hook face slot are the #1 choice at the elite level of motorsports (NASCAR, ALMS, DTM, Super GT, etc.). That said, there are plenty of World Challenge and Grand Am teams running StopTech discs with great success.

Disc hats
Both have a quality design and are made from aircraft grade aluminum. Both are optimized for weight and airflow.

Weight
I don't know the weight offhand, but I'm fairly certain the AP's will be a bit lighter.

Wheel Fitment
Wheel fitment is superior with the AP system as a whole.

Brake Lines
Both systems came with a high quality set of brake lines.

I actually ran the StopTech ST-60 system on my C5 Z06 while I worked at the company (and their ST-40 system on my 350Z). It is an excellent system with high quality parts. That said, the AP T1 kit was based with that system firmly in our sights. We tried to look at every piece of that system and produce something that eclipsed its performance, all while keeping the costs down. I believe we have succeeded...more technology, higher specification, lower running costs, etc.