It is a tough choice. I had the choice of putting my son in one of Dave's rentals, or in a rental SRF gen 2 this year. SM rental was cheaper, but I already paddock with an SRF, so I ended up buying a Gen 2, with the intent of also running it at Majors and bringing 2 cars to Indy.
I think of you Pro/Con the two, it all comes down to preference and feel. Locally here, both groups have big participation, and lots of people willing and able to mentor a beginner. Closed cockpit is a plus for SM, but really simple crash repair is a plus for SRF.
SRF 2 and 3 will literally run 17 heat cycles on a set of tires without any drop off, which I don't think is the case with SM. SRF has more car to car parity than SM since it is so highly controlled, but at the pointy end of SM there is a lot of car to car parity as well just through evolution. Sm is not really "spec" since it is not all single sourced and all the motrs are not turned and adjusted on the same dyno then sealed, but it is very, very close, and does allow a certain amount of tinkering not allowed in SRF. You can get DQ'd in SRF for the wrong oil filter.....
The Gen 3 conversion is 17K. I paid 13K for the Gen 2 I just bought, and it already had pretty much every available upgrade, so I'll be at 30K plus realistically 10 for "stuff", so the number comes in at $40K.
What does a true front capable SM cost? probably pretty similar all in.
You have plenty of drama BS is both class's, so that is a wash......
I will add that Mazda, of all the OE's does provide the best overall support and appreciation for club drivers, and that is a real factor for consideration.
SRF is sometimes called Spec wrecker ford, SM is sometimes called Wreck Miata. I don't think either is true in general, but both class's have their moments. Usually Mid pack and back. Miatas have dents, SRF has duct tape.
My advise, if you have the funds: Rent one of each, and go with what suits you better personally.