The Radi-CAL is the newest latest technology from AP available to the public. The Essex kit is tried-and-true race proven established technology.
The Essex kit is meant to have lower-cost consumables. The Radi-CAL has massive calipers, massive rotors, MASSIVE brake pads. I believe the Radi-CAL pad is ~40% larger than the older Stillen AP kit's (CP5555) pad, which I believe is a bit larger than the Essex kit (since the CP5555 rotor is 368mm and the Essex kit is 355mm). Nothing about Radi-CAL will be cheap to replace.
Radi-CAL also does not fit on some 18" rims such as Apex ARC-8. Essex kit will easily fit all 18" without issue.
Essex kit is probably a bit more convenient on pad changes because there's only hex-driven bridge bolts to deal with. The Radi-CAL caliper has two bridge bolts and two pins that have to be hammered out. And then hammered back in from the back side.
The Radi-CAL system with it's massive calipers, pads and rotors is designed to absorb and dissipate MASSIVE amounts of heat. But is it necessary on an M3? The Essex kit and the older Stillen kit (which I have) have absolutely NO trouble stopping an M3 lap after lap with no fade at all. They're not tomorrow's tech, but they are EXTREMELY reliable. At some point, you need to consider that having so much stopping power is gonna lock the wheel and activate the ABS. You can do that already with a high-torque pad in my AP kit as it is.
Also, I'm going to guess that the Radi-CAL pad shape is not very common yet. The Essex and Stillen kits have many many many pad choices.
I have 70 track days on my [older Stillen] AP kit and I've never had a hint of trouble from them---no fade, no failures, no warpings, etc. Finally, after 2 years I replaced the inner seals on my front calipers because there was a small bit of brake fluid seeping. But that's just normal wear from so much tracking.
So, if you want to track a ton and keep costs of consumables down, then the Essex kit is for you. If you want the latest greatest bling tech, then the Radi-CAL is the way to go. (I can personally vouch for the Stillen older AP CP5555 kit. Works great. I do the work myself to replace my rotors when they get worn. New AP strap drive rotors are a beautiful work of art.
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