Other than that, you save time where it counts. The speed you can reach with a walk-behind wont really matter. If the deck size is larger than what you were using previously, I believe this is when the choice becomes a personal preference.
The difference between sitting vs standing is how fatigued you get after each job. Sitting might make you lazy after awhile, but standing will at some point make you feel under-appreciated - if you can understand what I mean by that.
We are guys who dread spending an extra 2 minutes restringing a trimmer. Things like this just become irritating, because deep down inside we are all a bunch of a-holes. LOL.