stand on units are just as fast as your standard sit down ztr, some are faster.
the benefits for me in choosing a stand on unit over a sit down unit was that for one thing i'm standing all day. i'd much rather stand. i can remove myself from the unit quicker should i need to pick up an object in the way or in case of emergency.
i can also maneuver under low hanging limbs and things like that by moving my body, getting the mower into places a sit down won't go.
the size of the unit is another benefit. they take up less space than a sit down. so you can either have a smaller trailer or just have more room on your trailer.
this post is of course my opinion and what i personally think. i'm sure others will have other ideas. sit downs aren't for everyone and neither are stand ons.