I've had both and far prefer a walk on roof with built-in ladder. I don't go up there that often but just find it to be more convenient and worry free to inspect things and do routine maintenance. Also, when we camp in heavily wooded spaces, I sometimes like to go up and sweep the pine needles and stuff off top of the slides before putting them in.
Additionally, I put a cover on my trailer for winter storage and it's far easiler if I can climb up and down to roll-out and adjust the cover. When I'm climbing around like this to move and sometimes walk on the cover, I don't want to worry about making sure I step on the support beams or carry around wood planks to walk on.