the stander is a heavy machine, so compared to a walk behind, they will leave a little more damage to the grass. HOWEVER, you will only notice a difference if the ground is soggy. A walk behind mower will be lighter on the turf and be easier to handle on hills, but you will be beat up on the sulky and you wont be nearly as fast. A stander gives you the ability to turn faster and zip in and out of tricky situations. (think about vacuuming under a couch) with a walk behind you have to go into a tricky situation and be able to turn around or reverse with a sulkly, both of which are tricky and slow. a stander can go in and reverse out. This happens WAY more than you imagine.
It seems like you purchased the 54 on an impulse and realized it wasnt the right tool for the job. I did the exact same thing and paid for it later. We get this idea ingrained into our head that we need to have the same equipment that other larger companies have, so we can be as productive as them. this idea kinda bites us in the rear when we figure out that they have multiple commercial accounts and acreage to cut. my client base is almost all residential (3 commercial) so my properties are based in cookie cutter neighborhoods with lawns 3/4than acre or less. I still have a couple lawns that are 2 + acres, but we just double team them with the 2 36's.
Make hay while the sun shines!