Two things first: If your name is in fact K.Bacon, I guess we can presume you're not the famous actor, unless things are REALLY tough in Hollywood now
Also, you could change your name & username to KAwesome, as that would mean the same thing :bravo:
Anyway, as to your question - I had an 05 Grand Prix Comp G before my G6 and I really liked it. I'm guessing the 08 you're looking at is the base model? Probably a bit underpowered compared to the G6 as the GP is a slight bit heavier and IIRC the GP engine is a little less powerful. Much more room,especially trunk space, in the GP. Modability, I believe post 2005 GP's are tougher to get more than minimal gains out of due to a change in the PCM configuration. That was the case a while back; someone may have been able to crack them since then. But given that they haven't, that puts the GP & G6 on roughly the same par. The G6 GT should have TAPshift, which I really like but a lot of people dismiss as gimmicky. Also I'm presuming the GP doesn't have HUD, which was one of the great features IMO on my 05 and would definitely sway me to the GP.
So I guess the bottom line is if performance is your main goal (and it can't be TOO much of a goal or you'd be looking at different cars
), the G6 would be a slight winner, but if you need trunk space the GP is better. Pretty similar overall tho'