Posted for "DJ Molny" <djmolny@yahoo.com>:

 I have a 4-blade MTV-14/190 prop on my Extra.  It is smooth, but every time
 you want to work on the engine there's a #$%@! blade in the way.  Plus it's
 33% more expensive to overhaul.
 
 Everything else being equal, I'd opt for the fewest number of blades that
 can absorb the engine power.  Probably 3 blades in the case of a pumped-up
 IO-550.