The which was better: Messerschmitt Bf109e or Spitfire I is a debate that I have had with myself since childhood. However, perhaps the question should be about which engine was better, the Merlin or the DB601 (I will check later). Personally I suspect the German engine was better because it had fuel injection, while the Merlin had the notorious carburettor problem which led to the fuel cutting out and then flooding during negative g manoeuvres. Miss Tilly Schilling designed a partial solution with her famous orifice, but it took a long time to solve it completely. I think there was also a problem with the position of the glycol tank. The Merlin seems to have been better than the air cooled radial engines that were available, such as the Hercules. The Lancaster was faster than the Halifax. The Mosquito was faster than the Beaufighter. The availability of powerful engines very much hindered Italian and Japanese aircraft performance, while the availability of very powerful engines allowed American fighters to carry more weaponry and armour, as well as giving higher speeds and climb rate. I get the impression engine design was even more crucial than aircraft design.