My dad has an '05 6 speed GTO, I've driven it on many occasions. One of my good friends also has an '05 GTO. While the performance numbers are comprable to my SRT, the dynamics are very different. The GTO drives like a much larger car (it weighs ~3900 lbs) and therefore is more planted at speed. The SRT6 is much more fun in the corners, but less communicative in the steering. There is a lot to be said for being able to shift for yourself, but the GTO manual has a very long throw in stock form.

Personally, I'd like to have both, the GTO (as well as the SRT-6) got a bad rap when it was on sale, but I predict that it will be remembered fondly 20 years down the road. If you need a back seat, you can't beat the bang for the buck of an '05-'06 LS2 GTO.