I've built MPM's P-63A and C. The only difference being the strake added to the lower fuselage of the C. Here's a build article I wrote for the A:
http://www.pix.prettyneatinc.com/kingcobrabuild.htm The MPM kits come with resin and a vac canopy, and the Hi-Tech kits come with PE (I'm unsure if the canopy is injected). FM kits typically come with roughly molded parts, and some contain white metal parts and vac canopies. All are limited-run type kits, and require dryfitting, trimming, filling, and sanding.
None of these are easy builds, with FM's kit being the most difficult. The MPM kit is good value for the money, but suffers from inaccurate wheel wells (I didn't correct mine).
Hope this helps !