The 109F is the best turner, puts up decent climb/acceleration numbers, but doesn't have the speed of the later 109's. It also has the small calibre cowl guns, making them somewhat useless.
The 109G2 has more power, better, climb, somewhat less turn, being 400lb's heavier than the F. Still has the 7.9mm cowl guns of the F. The G2 is the second best performing 109 though, better than the heavier G6 but not quite up to the G10 levels. I like the G2 since it can still turn, but it can also climb well. You can actually take gondolas on the G2 and still have sufficient performance to maneuver...
The 109G6 is heavier still, turns worse, and has poorer performance overall, but offers two key upgrades. The cowl weapons move up to 13mm, making them useful, and the spinner cannon can be a 30mm cannon now, or the 20mm.
I tend to prefer the G2, some people like to turn-fight a little more so take the F4, people who like more firepower take the G6.
-Soda
The Assassins.