[Clockworkers Doll]

+1

This would be a good idea, just to be able to differentiate the two. Maybe even give the selected mode two different icons (just adding a shield over the icon for the spar), since I've seen a ton of people who just entered the arena have to ask if it's a spar or not, guards and normal people.
But the option for leaving the battle in general would just be a good idea, in the case of a different PvP style. Since then 'real' battles couldn't have it get abused.