Well, as an ex-owner of a 1999 Firebird V6 with the 3800, I will make a few comments.
1.) The F-Bodies platform was extremely, extremely outdated. The shifters weren't exactly great, with the manual transmission, and the quality was rather insufficient for what the cars were supposed to be.
2.) The G6 GTP coupe is a sleek, sexy car. The only angle I don't like is the rear 3/4 of the car behind the door. It looks too long, and makes the rear wheel look awkward.
3.) The GTP has a pretty damn nice shifter in the 6 speed if I do say so myself. I like how it felt in my hand. It was pretty accurate, though it felt slightly clunky.
My personal .02 is that the G6 GTP is a better performer than the V6 F-body, on the basis that it has about 40 more horsepower, and I don't think it weighs as much, though don't quote me on it.
The FBody will have more sex appeal, no doubt, but the GTP is a pretty nice car, and the 3.9L is a good engine, from what I know.