Bottom line for anyone who is considering the S2000 and/or the BRZ/FRS:
If you want to carry stuff, get the BRZ/FRS. Both are drivers' cars and both are very fun to drive when setup properly. Both cars teach you how to be a good driver. If you love what the Subaru/Toyota/Scion has done, get that. It's an incredibly easy car to drive and definitely very fun to drive at the limit (limits are lower on the OEM Michelins). Just keep the OEM Michelins for MPG purposes (you can get some wheels and nice tires for the extra fun times). If you love VTEC and a little extra rawness with the open top feel even though it's not that fast, get the S2000. However, driving the S2000 at the limit is much more challenging and requires a bit of seat time to get used to.
If you're talking about fast cars, there are 600hp Corvettes and 800hp GT-Rs that I have driven and instructed drivers on the street and on the track. Those are fast cars.
In the end, you can't go wrong with either!