I think the single races should be hardest, but that you should still increase the difficulty of the circuits, too. The circuit leader's path should probably stay rather hard, though, so that first place is actually challenging. Are you attempting to make it so warps aren't overpowered, or will those still ensure an easy win? Is there even a way to tone the warps down?
So you vote for single races or circuit races? I'm not going for both at the moment, that would take even longer. I had already thought of having the boss racer a bit faster at some points, but even without it is challenging. And my aim is to not exclude the use of warps and shortcuts, I want to give the player full freedom and not having him/her restricted to such rules as no warping allowed.
Oh, it's one or the other. I guess in that case I would prefer single races.