After totally coating the front of my trailer with mud just going up a friend's driveway last week, I started looking at the different types of mud/rock guards/flaps available.  There seem to be two main types:

1 . Mud flaps - permanently attached just behind tires.

2. Mud/rock guard - temporarily attached to back of truck and goes across the back (some are two flaps and some go all the way across)

My first thought is that I prefer the first type - once they are installed, I can basically forget about them.  They are also a LOT cheaper. 

My question is about performance - does one type work better than the other?  Differently?  Thoughts?

Thanks