You can't go wrong either way. It comes down to personal preference. I'm partial to the 86 platform after coming off of 3 years of Miata. My brother runs an'06 mx5, coils, bars, 235's and a couple of performance mods and It is an amazing car on the track

I moved from an SR20 powered S13(240sx) to my 2013 FRS. I got the car not knowing about autox at all and heard about it from another owner. The option of the ND wasn't a thing in 2012/2013. I do think it would be fun to drive one, however the several that I saw start autox this year did not last many events due to transmission issues. Although there is one that is still going strong in CS locally.

If I had to stat all over again then it would still be the FRS/BRZ due to SSC. I think its gonna be a fun class. Knowing that STX is just a hop and a skip over from SSC is nice as well. Being able to fit all your tools and wheels in the car is also a plus. However it got annoying and I decided to do a tire trailer.

I think its to early to get into an ND if your looking to move to ST classes. There just isn't enough info on setup yet. The twins have a ton of information for STX and plenty of routes for a build. SSC is also almost completely set and you can get the parts right from tirerack.

Either one would be a good choice but if going to ST* trim I would say a twin is currently the way to go. Also an STX prepped twin is a blast at the track.