Terry,
Both planes are great flyers. The main difference is that the RSM Mustang is larger by about 50 square inched than the RSM P-40. Also the P-40 has my thicker airfoil while the P-51 was requested by Eric Rule to be built around the Charles Mackey "Lark" airfoil. Both airfoils are good, but the thicker P-40 airfoil has a little more lifting potential, I believe. That being said, It would take a top notch pilot to tell the difference when the planes are flown within normal flight envelopes. Also as a new offering, I just finished a design for a new 40/46 size Mustang which is the same size as the P-40 (about 560 squares). I have a laser to cut parts for short kits if desired. Of course, I can supply plans to anything I have. Feel free to get in touch at the E-mail Mark listed.
Pat Johnston
Design Staff
Skunk Works