Is there a consensus regarding the benefit of one over the other (ignoring cost)? Is my concern that an adapter will move the lens too far away from the negative to achieve focus justified?
I have that mentioned ring (32.5mm to M39 Leica mount ring). It does not change the distance between lens and negative. It just "fills" the gap between the inner and outer thread. The more expensive adapter will add some distance, probably 2 or 3mm. I would get the ring.
The advantage of another lens board is that you can dedicate a board to a lens and swapping lenses is less fiddly.