I cannot speculate about the future market values but I can say the 993 and 997 are the best two models that Porsche has ever made and within them the 993 tts and 997 GT's are the best of the 997's and the market will probably reflect this going forward.

I do suspect the 991's will eventually make my list but they have not grown on me yet. They are a little larger than I like my sports cars to be.