AP Racing CP8350 (Essex Sprint kit) vs Stoptech ST40 Comparison
 

We've had some requests for a comparison, so I took one of the ST40 kits in stock out for pictures. Please excuse the quality, I took these with my cell phone.

We chose the AP Racing/Essex "Sprint" kit for our car, and you'll see the reasons why below.


Comparing calipers

The ST40 is larger, and has dust boots. It's also painted. The AP Racing caliper is lighter, and more compact. It's also wider. Notice that our (well used) caliper has no discoloration.
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Comparing pads
The ST40 pad has more surface area, causing more pad drag. The CP8350 pad is significantly thicker, and happens to cost less. Note that the backings for these pads are the same thickness as OEM. The pads are BIG.
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Comparing rotors
The ST40 rotor is 328x28mm. The CP8350 "Sprint" rotor is 299x32mm. Weight is approximately the same, so heat sink capacity is theoretically the same. Notice the CP8350 rotor is thicker, and has more vanes, giving it significantly more surface area for cooling. Both rotors have directional vanes, and are significantly lighter than stock due to the hats.
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There are a few things you didn't mention.

Even though they weigh the same, there is a benefit to a larger rotor- it takes less clamping force to provide the same brake torque. So it's possible you will get better brake feel out of the stoptechs.

The rotor attachment is also different between the two. That AP sprint kit is fixed while the stoptech has float. So less pad knock back and more even pad and rotor wear. And it wont fail like that set of wilwoods everyone has seen.

As for the pads, can you quantify "extra pad drag?" The st pad seems to be a similar volume as the thicker ap pad, so they might still last the same amount of time.

Anyway, both great kits and you can't go wrong with either.