I honestly don't see why you can't mix/match.

I like a good main plot with "monster/dungeon of the week" thrown in every once in a while.
It worked for X-Files, no reason it it can't work for me.
Your games don't need* a 100% consistent and back traceable level progression.

I love having holiday themes side quests throw in.
You were in the middle of a dungeon? Well today is March 17th so now your group happens to be walking threw the forest and you spot a rainbow and legend has it, at the end of rainbows . . .

Next session, back in the dungeon with any extra items/levels.