If you can get an exmark with the ECS handles, It is far more comfortable to use than the normal pistol grip handles. Scag also makes a nice T handle unit as well. Otherwise they are essentially the same machine. I think the scag has an electric PTO while the Exmark has a manual lever for the blades. But like I said they are pretty much the same machine.
I actually have an exmark 36" and a Scag 36" walk behind as well as a Scag 48" walk behind. The Exmark is the oldest and it is still the prefered choice of the crews that do heavy mowing because of the ECS handles. Hope this helps.