The best way to be competitive in our region is to prep your car close to the limits of your chosen class. You really need for your car to be somewhat competitive in that class as well. The best examples of this are Brian's STi, Ry's Mini, and my M3. Of these cars, in my opinion only Brian's is currently capable of winning a national championship (because it's both well-prepped and very competitive in its class).
STX will limit you to a 245mm tire on an 8" wheel. You've got a lot of car for that much tire. STU limits you to a 275mm tire and no wheel width restriction. If you go STU, you will absolutely want to be running that maximum tire width on a big enough wheel (I'd consider 10" to be the minimum). All ST classes will require some degree of emissions compatibility and will not allow for update/backdate among model years, so if that's in your plan you should probably look to Street Prepared. Street Prepared has additional allowances over Street Touring. If you don't plan to take advantage of those allowances, the main reason you'd want to run ESP is to be able to run in the street tire division locally.
So, long story even longer, if you're dead-set on moving from FS, I'd prep to STU rules and then run either STU or ESP. But be warned, if you make the class switch half-heartedly, you'll find yourself uncompetitive. When I moved from BS to STU, I did so by doing coilovers, bigger wheels and good tires, an aggressive alignment, and race seats all at once. If you don't take advantage of the allowances in a class you'll end up in the land of the uncompetitive car.
Mississippi Region #83 SS / #97 STU