The Champs I have thrown were pretty much dead straight with a sweeping fade, and I throw these types of discs ~350'. Compared to a PD, the PD is straight but feels like it has more HSS in reserve, and the PD fade is more at the end and stronger...where the Thunderbird sweeps a bit more. The champ Thunderbird just feels mellower a bit.
I've thrown a beat up G star and it had tons of turn...I threw it flat and it ended up 20' right of my aim. Not sure how long/abused that one was, but if you want to keep the disc in your bag a long time for the straight type shots, the G star may get pretty mellow. I bet the G star start off closer to what you want though.
I agree though with what was mentioned above...if you already have a Teebird and PD and are throwing these ~325', there would be significant overlap with the Thunderbird I think. I imagine people who can throw fairways 400' could see some very real differences between those discs though. But seeing as you have those other two, the more mellow G star Thunderbird would likely overlap less with your PD (unless your PD is beat and has some turn, or you plan on replacing with the Thunderbird....or you don't care and just want to throw some discs which is fine too).