I would fly WWI planes if there were a decent flight sim. Red Baron 3D was one that I used to fly. The better guns, higher speeds, and higher performance of WWII aircraft seem more popular than the slower, twin (or single) .30 calibre, slow climbing cloth and wood birds. Saturation bombing wasn't available in WWI, so that's an issue affecting popularity as well.
Another drawback to the WWI planes is reliable data on aircraft performance. The tactics and maneuvers, however, are the same for both era planes. Bolke's dicta applies just as well to WWII aircraft as WWI.
Jet aircraft are another option, but that doesn't work (from my experience) until someone comes up with a clever way to deal with internet lag. Given the choice I'd go back to WWI Spad's, Albatro's, and Camel's. But then, I'd prefer a "dogfight only" arena.
Regards,
Malta