Your decision should be made on how many autox events you plan on attending.
How many events do you want to do, how into it do you want to get?
A s2000 will be much more forgiving autox than the 5.0. It will also be in a different class and have different competition.
Now if you mod either one you will change classes and have harder competition. Thus a supercharger 5.0 is not much of a autox vehicle but a 5.0 w/o supercharger and with several $$ in suspension, brakes and tires might.
I had a friend who had the bare minimum mods allowed done to a s2000( kn drop in, brake pads) and would pull some jacks and race tires and tool box on a tiny trailer behind his s2000 to auto x events.
I've done a few autox events with different cars and I will tell you I had the most fun and best times with a miata with tires and good brake pads. This is because the autox I did wasn't on tracks but cone setups where you never really reach high speeds 1-2-3 gears. A 5.0 with a supercharger or even 500hp doesn't have much of a 1st gear. With that said if you plan on doing like 2-3 events a year total then by all means don't hold back and get that sexy 5.0 - preferably in that bright blue with black accents.