Generally, there isn't a 'best part' for an aircraft, it's really just parts that can most benefit your play style. But firstly, no, don't buy either of those aircraft unless you need to get into the section of the aircraft tree behind them. Secondly, the few parts I would say would most benefit those aircraft are Pitch and Mobile Fins for -Pixy-, and Armor and Awareness for the -Black Duck-.
I have both the T-50 and F-22. Really depends on you. Neither had a ton of parts slots to work with, so choose wisely if you go with one. I say that because you don't want to spend $673k(or something like that) on one and absolutely hate it. The PAK-FA has great mobility from the start, but I went with the F-22 because at level 5 its ATK DMG (AIR) star is at A+ along with the speed, with the mobility at just A.