The P-63 Kingcobra (or P-39 Airacobra) has been the most popular Slope Scale kit over the years. The flight performance is just a little better than most any other PSS plane I've flown. The reason for this is hard to explain. The design differs little from most of the other kits, yet it flies better.
The kit features a fiberglass fuselage with a separate hatch. The fuse is designed so that it can be used as an electric or as a sloper. The large hatch covers a flat tray that can be used to mount batteries or ballast. The wing core is a highly modified RG-14.
The best flying weight for the Cobra is between 30 and 50 ounces. Although with a light layup fuse you can build this kit around 20 oz if you're careful. The electric version will weigh about 30 oz with a small brushless speed 400 style motor and a 3S 2100 battery pack.
This is my electric cobra, 29oz ready to fly with a 2100 3S lipo. I run a Mega 16-15-6 motor. The performance is great with this setup. It's not ballistic but fast enough to be fun and easy to launch and fly.
slope scale p39 at Cardiac from Tee Efelgee on Vimeo.