The manual versions of the Vantage (V12) (420 bhp) and the GT (435 bhp) V12 are available in the US but are very rare. not sure about the i6 supercharged.
I think that it would be better to go for manual over automatic, especially if you are an enthusiastic driver. So far all the literature I have read indicated that there is a fairly large disparity in the performance between the two different transmissions on both the 6 and 12 cylinder varieties. By getting the manual I feel you will get much more car for the money.
On the 6 vs the 12 cylinder version... Each has own small advantages over the other. I believe that the 6 is not subject to rust as much as the 12, but the 12 has a somewhat better build quality being the latter car. The 6 also has a supercharger, something that would require additional care. Given your choice I would probably go for a 2000-2001 Vantage coupe (manual) or the later DB7 GT (not GTA).
There was a recent Autocar (magazine) article on the seven and I suggest you take a look at that.
Good luck