1 vs 2 Fans on Water Cooling

I'm looking to get a water cooler for my CPU (and possibly GPU) and was looking at the closed water coolers on different websites and there's usually about a $40 difference between 2 fan ones and 1 fan ones. What is the cooling difference between them? Thanks!
 
What do you wish to accomplish with water cooling?
What cpu, gpu and case will you be using?

Water cooling is really air cooling, it just differs on where the heat exchange takes place.
Water cooling requires a radiator with good fans to be effective.
All else equal, more and larger radiator fans are better.