go with an '05/'06 GTO. my buddy was dead-set on buying an '08 mustang. he took it home, test drove it, showed the wife, etc. etc. he drove it over, and i let him take my '05 GTO out. needless to say the mustang went directly back to the dealer, and he got an '05 CTS-V, which is comparable to the GTO.
the '04 GTO's had the LS1, with less hp, but they are cheaper