To prepare for FSO we did a bunch of 2vs2 duels with the 39D and 109E. I was impressed with the 39D and believe it's a better fighter than the 109E (if you take the HS 20mm). The 109E is better in the vertical, but good luck gaining enough separation to use that advantage. Other than that, the 39D is faster, rolls better, and holds energy well enough for it to take a couple of full circles before the 109E's turn rate advantage shows up. The big weakness of the 39D, of course, is acceleration and sustained climb.