996 Turbo can be made into a nice street/track car and if you consider the cost of a the BRZ, I think you'd be money ahead by sticking with the Turbo.
However, I have a 993 Turbo that is track prepped very similarly to my BRZ and can say the BRZ is more fun and has improved my driving immensely simply because I feel more at ease driving it at the limit, and to be fast, you have to drive it at the limit. With a Turbo, it is all too easy to lean on the power and AWD when all the fun is in the corners! So, for that reason, I'd recommend the BRZ
Also, I recommend never going to R compounds for HPDE. The 200TW tires are only a bit less sticky, but they communicate a lot better, last a lot longer and are much more street friendly; thus eliminating the need for changing tires between street/track. AD08s and Star Specs are quite good, as are several other tires. I used to run R comps, but now only run these; way more fun!