Usually non-FIA seats need to be attached to the roll bar like aluminum seats, or they are not allowed at all for some events. I would not get a seat without an FIA sticker if your intent is to do actual track events.
I've had good luck using MOMO seats in both Miatas and MR2s, even driving them on the street when needed. Also, not sure where the only use a side mount came from, we used to run endurance cars with bottom mounts to aid with the difference in driver heights with no issues, one even doing 4 or 5 end over ends at speed with no seating issues (driver stupidity not withstanding). They have to be mounted correctly, and we used the manufactures recommended sliders. Of course the rules may have changed in the last few years, and the side mounts probably are a bit safer.
That said, I do not, do not like aluminum seats at all. A few years ago at Daytona we were in a huge pile up in Nascar, the top of the seat was pointed one way, the bottom the other at maybe a 70 degree angle. No popping back into shape, just bent and staying there. It put the driver in more than a little pain. I will never ever use another aluminum seat after that.
A Miata may not can generate enough energy to do that, but I'm not sure I want to take that chance. Just like other safety gear, get the best seat you can afford. Once you get the seat, make sure you mount your belts properly as well. If you don't have enough experience, get someone to mount both the seat and belts for you, or look them over after you are finished to make sure everything is correct. I can't stress enough how important both of these are.