Yes, depending what those differences are, that should determine which way to go. You will probably want an indoor space, which might mean a gymnasium or otherwise a room in which they’ll let you ride unicycles. That might put you into the sport category.
Society may get more non unicyclists involved as you could pitch it as a come and try group. Sports has the advantage of easier use of the sports facilities.
It does depend a little on the university though as some require a higher fee to cover Sports societies. Worth talking to the SU and see what the main financial implications of each is.