If i were you i would Get the G8 GT. I know this because i was in your spot in 2009 and i got the G8 and i love it to death. ok here is the differance between the GT and GXP
GT 6.0L L76 with L92 heads and intake has DOD that shuts off 4 cylinders
GXP 6.2L LS3 with L93 heads and intake and is availible in a 6 speed manuel
The DOD is not very good. Its subject to failer. IMO the weakest link in the car. DOD delete is about $1500 do it yourself or $3000 done for you. The GXP is nice and nothing really needed to be done. So if you find a good deal on a GXP take it. If not the GT with the DOD delete is right there with it
Oh brakes the G8 has smaller brakes than the GXP not a factor unless to slam on the GT at 100 or better then you get brake fade.
replace factory rotors with slotted and dotted from brakeperformance.com and no more fade. Also take this into thought ....
HUH thats all thats hitting me right now Both are GREAT cars and react well to mods so enjoy.
Anything esle just ask.