Pretty much the same IMO. I don't have data to back it up but just go with the K springs unless you don't want the extra .5" lower. I couldn't find the rates on the P springs but copied the K springs info below and also a brief thread I saw on SVTperformance
Spring Rate: Front: 228 lb/in.
Rear: 199.5 lb/in. Usage: Street
Year: 2005-2014 Model: GT Coupes
Progressive or Standard: Progressive Estimated Drop: 1.5"F / 1.5" R
2012 black on black GT Premium Auto 6R80 - 400A
20 x 8.5" AR Torq Thrust M - BMR springs & Koni str.t shocks - GT500 mounts - J&M LCAs - CHE reloc brackets - J&M panhard - General GMAX AS3 - GT500 axel backs (no resonators) - GT500 spoiler - MMD tail light trim - Roush side splitters - AM upper/lower billet grill - Bama 93v2R tune - Airade CAI
i recently had the P spring installed with the koni strts. i too looked everywhere for info on both the p and k. from what i gathered is they are about the same (?). I didnt want big drop, just to close the gaps a little and get rid of the nose dive/lift. The p springs went just about a perfect inch. ride and handling is muuuuuch better than the stock set up. Definitely stiffer but not jarringly. No more boat on a wavy pond feel when going over bumps and humps . im going to guess that between both, it will depend on how low ya want to drop. 1/2 inch makes a difference especially if ya have a steep drive way like me. If you keep searching youll see a lot of people think they are made by eibach but i dont know.
2013 Mustang Gt Premium Red candy metallic, Comfort pkg. Securty Pkg.
-Magnaflow competition catback
-Koni Str.t shocks/strut, Frpp p springs/bump stops, BMR adj. panhard bar, BMR lca's, BMR lca relocation brackets, ST suspension camber bolts
-MGW Race Spec