I've got the the roadster version of the cayman (ie boxster) and will say that I love both of the cars.
Things I like better about the BRZ:
1) The steering wheel. Absolutely fantastic
2) Short shifter (my spyder was not equipped with the optional short shifter)
3) better gas mileage
4) better daily driver
Things I like better about the boxster spyder
1) exotic looking and rare
2) flat 6 sound. Nothing beats it
3) Performance.
4) better weekend/track car
Both are fun to drive on the street and the track. There's no doubt that the boxster/cayman outperforms the BRZ in every way possible, but on the street and everyday driving it's really not that important. People like to bag on the BRZ interior, but I honestly think it's a great interior. Deviated stitching everywhere, great seats, good layout and positioning. I don't get out of the Porsche, sit in the BRZ and say "gee, this interior sucks". No way. If I had to choose between a cayman and BRZ, it would be a Cayman without batting an eye. However, both have a distinctive and important role to play in the automotive world.
