Imo, a vertical rig is easier to work with, if it fulfils a few requirements.
- Having the scanning surface flat on a table with lighting installed underneath the tabletop makes handling very comfortable because you don’t have to stretch and hold your arms while doing delicate adjustments
- Stability: Get the heaviest table and mount things directly to it. Prevent flexibility by NOT getting a flimsy column - or mount structures with robust parts.
- Adjustability: Add a means to adjust alignment, e.g. with a geared head instead of something that is friction based like a ballhead.
- Tethering: Prevents you from having to get up for live view.
A horizontal rig has its merits too, but stability requirements increase with heavier and longer lenses. Also, there is a higher risk of kinking your film unless you have a means to hold the loose ends e.g. of uncut film.
The prototype I show here and here can serve as a vertical rig as shown in the post - and it can be flipped backwards to make it horizontal. Observe the following: Heavy duty material, a short column and the lens support. I plan to further develop the rig though. Shielding stray light is the topic I’ll have to deal with.