we've got a side bed as we dont want a longer van,we go to France and some pitches are tight with a big van.Personally I'd like an end bed with a center shower toilet but this layout is only on a long van as far as I know.A transverse side bed gives you the option of walking around the bed with a shorter van but there is not much space to get round as the vans are usually 7'6" and the bed is 6'3" plus the thickness of the walls .Some manufacturers make the bed with a removeable pice of mattress so you can shorten the bed in the day .Its not really a thing where there is a best but more what suits you,we live with the nuisance of getting out of bed for the other person to make tea use toilet etc to keep a short van,if I had to have a big van for a family I think I'd go for the end bed.
Hope this helps you
Best wishes