Depends on conditions and dealer for me. On ZTRs scags are faster hands down and can handle a wider variety of conditions. In a perfect weed free dry lawn Exmark will leave the best cut period. If I were you I would go with scag. I have been running Exmark for years but will be getting scags or ferris mowers to replace them.
Knowledge is knowing a tomato is a fruit, wisdom is not using it in a fruit salad.