i think overall its still comes down to pilot skill, both mustang and spit are capable fighters with their own advantages. most of the good pilots fly for years so its no wonder u get shot down after just having a couple of flights. dont let it frustrate u, try and learn out of your mistakes and accept to be shot down by "aces", its not a shame at all. sooner or later u will have your first victories and the fun starts!
good hunting