The higher level dungeons will reward more XP, and as you have noticed, the higher level you are the more XP needed to level-up. The Dungeon Roulette adds a bonus to your XP, depending on the level of the dungeon you get, that essentially makes up for any XP you would have gained otherwise. For example, if the highest dungeon you could queue for was Cutter's Cry (Lv38), and your roulette gave you Cutter's Cry, the roulette bonus would be very small, but if roulette gave you Sastasha (Lv15), then the bonus would be very large.

To answer your question, it would be more XP to queue for your highest level dungeon when you've already gotten your roulette bonus. If your under level 24 or so (and don't have many dungeons unlocked) then your roulette may be faster, as you will have a shorter queue time, while the XP difference at that level is minimal. To reduce queue times a bit, you might also consider queuing for your top 2 dungeons.