Not really a very good comparision, but given a choice between the two I'd say the V8. The power will be available at a much lower RPM and as a result the engine will hold up together better over time. However, someone buying either the S2000 or GT Mustang isn't interested in endurance, anyway, so I guess it doesn't matter much.
To put it into better perspective, would you rather have a 400hp engine that you only used 50% of it's available power or a 200hp engine that you used 100% power? The 400hp engine, while more expensive initially, will last you quite a bit longer and give better gas mileage than the 200hp engine that has to be wrapped out WOT all of the time.