You probably saw my other post about the caliper, but I got the 15. Currently using with stock caliper, and it is a bit wooden. It works great with stock caliper if you've got a nice pad and rotor set. Steel lines as well.
The 15RCS is designed to work with their aftermarket 4 piston caliper, and the 14RCS is the bolt on upgrade.
Depends what caliper you want to run with. The stock caliper isn't bad, I've seen many club racers use stock 300 calipers up front. That with the 14RCS is a great rad combo with a lot of potential.
But if you don't have ABS and want the absolute premium in brake power for these single rotor machines, 15RCS and the Brembo 4-piston caliper would be end game brakes.
That said, I like my 15 enough with stock caliper that I won't go buy the 14, since I found my magical pad set match. Standard HH pads from Galfer or EBC I kept blowing up on track and they felt bad. The EBC EPFA half track half street pad is the only pad set I've found to work with the 15RCS and stock caliper that provides the best feel, actual stopping power, and never overheats. I am hard and late on my brakes, so this setup works great for me. Just a bit hard on the lever pull, but it has taught me a lot and prevents me from locking the front ever.
Here is the EBC HH pads I blew up. I broke them in and rode them around for a good amount of street, then 2-3 sessions of track in they rainbow'd and shattered, and squealed insanely loud in the pits. Never again. EBC HH pads aren't bad, but they just absolutely did not work for my setup and style on track. Nothing was wrong with my calipers either, I rebuild them often.
Sorry for the ramble, hope this helps.