There are a couple of possible answers, depending on what you're asking. By running a larger combination you will get less drivetrain wear and possibly a smoother feel. You also will have smaller ratio changes if you change a tooth in either the cog or chainring. (changing from a 20 to a 19 tooth cog is a 5% change, from an 18t to 17 is a 5.6% change not much, but noticable. There is a bigger effect in chainring size). By running the smaller set-up, it will be lighter and you'll have a little more clearance so you won't have to roll your pants as high.
As for the greatest common factor, which isn't really the right way to describe what I answered above, that will effect the number of skid patches. The number can be figured out by dividing the number of teeth in the cog by the GCF. So with 52x20, the GCF is 4. 20/4 = 5 skid patches. So when you skid, there are 5 possible spots on the tire that will be on the ground. 10 is you can skid with either foot forward. with 47x18, since 47 is a prime number, you will have 18 skid patches. So there are more spots on the tire to be worn down by skidding and the tire won't wear out as quickly.
edit: i took too long to type. In the time it took me to write that, yoshi beat me to it.