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.