For me it isn't about finding some correct number, but about the speed of progression and quantity of content per level. Quality matters too, of course.
My games are fast paced, very high on content, but very, very slow in leveling compared to almost everyone else. Each level of every class is jammed packed with content. However, advancement upwards is dependent on each player for what they do, even though I set the granularity of the bar. Plus, adventuring usually gains characters power that isn't tied to class level at all, so every moment a gain or loss may be had.
I like long levels as it makes it very hard to grind through a level without learning a large degree of mastery of it. Higher levels presuppose that player ability, so grinding is not only boring, but detrimental to future ability to play well.
Merging levels and content, making XP a dial, and leaving it up to each individual campaign is probably the best route to satisfy the majority. I can weld 1+1 levels to = 2 as a single level. Higher numbers at high levels can be a PITA, but with a clean system even that can be dealt with.
