Sports cat will pass MOT, decat won't because..er....of the lack of a cat! (fail emmissions test)
Decat will give the most performance gain and arguably sound better (louder at least), but is more hassle as you have to take it on and off every year (not too bad if can do yourself).
Sport cat less hassle - thats why I'm getting one tomorrow
Decat is alot cheaper though (its only an empty pipe after all)