The marketing tool for verticals is they use a bit smaller footprint when being stored. Some folks like them for that reason.

Off side is any work or maintenance needed on the pump is less easy to do.

There is a real disadvantage on vertical set ups when talking about gen-sets. I add this for those considering them. Often they are ran on dirt. Gen side sucks in air for cooling just as the engine does on gen-sets. When the gen side being below and next to the dirt when ran that way, they suck dirt into the brushes and cooling system. Lot of excess wear results.
This isn't really a factor on pumps/pressure washers, only the more difficult repair/maintenance due to positioning.

Year 2000 Sears Craftsman pressure washer here with vertical shaft..still going strong after replacing unloader valve this year.

Are you going to be using this machine often? If so save your money and buy a Landa or a Hotsy brand. Northern Tool also has some decent cold water units. Try to get something belt driven. Craigslist would be a good source.

Those units at sears and home depot are throw away units.