MM2! (Ducks and runs....)
Actually, it depends on which question you want answered:
Which monsters book do you use more often? (if you have both)
When I cut my teeth as a DM, I used the MM2 a lot - much more than the FF. Thing is, the reason for that was mainly that my friends/players already owned, or at the very least had memorized, the Fiend Folio. So when the MM2 came out and it wasn't available at the library and my friends didn't buy it, I was actually able to surprise them.
Also, it is a very useful/functional book - that doesn't have the Flumph, but does have the Tarrasque.
However, getting back to your core question...
I found a copy of both for almost the same price (1$ difference) and am planning to pick up one at least.
So which do you prefer?
You've gotta get the FF.
Here's the thing:
The FF is whacked - but it's inspired in its whacked-ness. The monsters run the gamut from great to goofy to weird. And it has fantastic (if sometimes goofy) art.
If I'm looking for inspiration, this is a book I'll sometimes pull out and reread.
By comparison, the MM2 is a very pedestrian, useful monster book - that I never feel like reading.