Having had an '04 GTO, then an '06 which I traded for my GXP, this is an interesting question. All mine have been M6.
They for me are different beasts.
Stated as simply as I can think of, the GTO is a more visceral, emotional, in-your-face car. The GXP is more refined, sophisticated, classier, and athletic.
And comparing the M6 versions, my '06 GTO was just a tad faster than my GXP. It weighs about 250lbs. less and gives up only a few ponies. Had mine been the automatics, the GXP would handle the GTO.
I love them both for somewhat different reasons. Having 4 doors and a REAL trunk coupled to a monster motor sealed the GXP deal for me (family and hauling more than a duffel bag). I am happy with the GXP upgrade, but I do miss the Goat though.
As 'exclusive' as the 1828 GXP club is, there's very little G8 camaraderie on the road. Probably because there are so many more out there with little visible differentiation between our 3 versions to 99% of people; the G8 breed is a common (but really good looking) car. And the G8 was marketed as a sporty family sedan rather than a muscle car. Many bought them as sporty grocery getters rather than enthusiast transportation.
In contrast, my experience driving the GTO and seeing another always meant a light flash or wave. I also got a lot more looks in the Goat, both the 'What is that thing' and 'Wana go?' type stuff.
So if your pocket book can handle a GXP, it's a better overall car that will probably last longer. It's built a bit better, and has more 'stuff' in it. The GTO is more emotional and its faults are somehow more endearing.
I do think there's an easier upgrade path with the GXP with bolt-ons. The LS3 left a fair amount on the table versus the LS2. You can easily bolt on 50whp with the GXP. The LS2 in the GTO with the same mods (CAI, headers, exhaust, tune) might get you 30-35. And the trans tuning on the G8 automatics is light years ahead of the GTO.
You won't be disappointed with either, you just need to decide which is 'you'.