Front loading kilns are more preferred and often more suitable to be used by traditional potters. When stacking shelves in your kiln it is easier to do so in a front loading kiln. It is often less of a strain and much easier for users with back problems.
However top loading kilns are are common in industry. With a top loader kiln you can look from a bird eye view at your kilns shelf and see all of your pieces of work and distribute them evenly with no blocked view.
With a front loading kiln whilst you stack from back to front your view is blocked as you load.
Top loading kilns are open and closed with a lid whilst front loader kilns are opened and closed with a door. Often their can be more damage / general damage caused.
Enamelling kilns usually are front loading kiln, and there is typically a window or an opening to view your pieces as they are in the kiln.