worse case, it would be pretty easy to convert to upright. might have to beef up the 'end' that it stands on, but 90% of it would remain intact...basically change the door.
for long, narrow items, i thought the horizontal would be easier. mount the part on the trolley, as opposed to hanging it like a trophy fish
both ways still have their downside. on the trolley, it may be difficult to do the underside as it will be low to the ground. hanging, it may be hard to get the top and/or hang the item if it is a long.