I think there are two scenarios here. Either you want to do a Kirishima cosplay, and keep it at that for the My Hero’s Academia franchise. Then it is a good idea to do the combat uniform as it stands out more and is unique to Kirishima himself.  If you want to do multiple My Hero Academia cosplays, then I recommend you to buy the school uniform. All the students wear this, so it is going to save you some money (and closet space). It is of course a bit less unique than their combat uniform, which is special to every student.