funny, I was just today reading some old '07 GRM mags in my garage that were when they were doing the MGBGT Miata drivetrain setup....
owning a GT6 an having driven a couple BGTs (non-V8), they're both good options depending on what you want. Stock for stock the GT6 has a peppier engine and (IMO) a better handling car for high-speed work (the GT6+ at least). The BGT on the other hand has a superior ride, more balanced handling, and a more "modern" interior (if you could say that).....
I think the GT6 is more fun to drive on windy roads while the BGT is more fun to drive everyplace else, basically :)
But with a Miata drivetrain swap, you get the extra power and still have the nice handling balance and interior on the BGT, so best of both worlds.
Frankly, i wish I had one of each but the wife won't allow it :)