how much did you get it for?
I'm really leaning towards the Miata. From what I'm understanding, the older corvette would be faster if I were to race the two cars on a track, but on mountain roads, I'm not going to be driving the car at it's capacity. Instead the "telepathic" handling of the miata will come into play more often. Also, the ease of operating the top on the miata, the less dated and more refined interior, and the more high tech and rigid chasis on the miata will probably outweigh the benefits of the corvette. Other than raw power, I can't really think of any other benefit.
The next decision is whether to stick to the 2006-2008 miata or go for the 1999-2005 miata.
EDIT: By the way, this is hilarious:
http://www.youtube.com/watch?v=0rgr69YYl3w