Saturday is the day to go, sunday you will get only 3 runs before they go into the elimination which takes only the top 64 places in PAX. Saturday you will get more seat time for sure unless you are in the top 20 or so of PAX.
Miata's are fun cars that even after 17 years are still very competitive in every motorsport they are put in. You have a lot of places to run the car from CS to CSP, SM2, not sure what Prepared class they run but theres always competition anywhere and a ton of aftermarket for them.
It for sure would be a more fun car that the Saab!!