quite broad question. cannot give straight answer.
if yr looking for 2 seater,, s2k is good choice but if 4 seats is a must, rx8 is it.
if yr looking at horsepower figure ,, do the horsepower /litre check and s2k has more than standard rx8.
road handling is good in s2k , and isnt too bad for a 4 seater rx8. rx8 takes the cake from any 4 seater cars in terms of road handling.
fuel economy. no too sure but s2k should do better mileage than rx8 if driven easy.
boot space. interior space,, rx8 has more.
engines you cannot compare, one is 4inline other is rotary. rotary is expensive engine than a piston engine.
and so on... the comparison is pretty indirect . no direct comparison can be made because both fall in different categories.
this is my opinion.. may differ from yrs, thx