WS6 is faster, unless you are able to purchase the '05 Mustang Cobra which I don't believe it available yet. But the 2002 Trans Am is used, who knows what kind of abuse its seen. I mean, come on.......what trans am hasn't been dogged on? Plus, I think the new '05 Mustang bodystyles are pretty sweet looking. Get more looks with that one.
Both cars have TONS of performance parts available to mod. So it's really up to you, but i'd go with an '05 Mustang.
Pontiacs are sick. Mustang are ok. The 05 Mustang however blows. I went and looked at them, and I didn't even want to test drive it. The outside looks pretty awesome. The inside sucked. I woudn't buy one. I do like the last Trans Am and the new GTO is even sweeter. That would be my choice, GTO.
I don't think he wanted to know if 'Pontiacs rule' ...Sigh...
The LS1 engine in the WS6 is without question one of the best V8 engines ever made, and the fact that it can still use reliable/bulletproof technology like pushrods is perhaps its best trait. These are engines that can take abuse for over 120k without any problems (car may have issues though...still a GM) and are un-equaled in bang for the buck, and mod potential.
As for the 4.6 in the GT. The old 2v 4.6 has had some problems making big power numbers in the past. Additionally the lack of displacement has been an achilles heel in the past as well. The new 3v design has supposedly improved on some of these things, and recent tests have put it at about 13.5 at 103 which isn;t a bad trap speed at all.
But in the end, I would go with the LS1 engine without a doubt. Stock WS6's routinely trap in the neighborhood of 107-109, and headers/intake/ 112. Not to mention the heads/cam guys that are routinely in the low 11's high 10's while still having a streetable combination.