Originally Posted by AlmostTrick

Tests have shown larger rings and cogs to be slightly more efficient. They also last longer because they don't wear as fast as smaller ones do. The only drawback, and it's a small one in my opinion, is they are slightly heavier.

Originally Posted by JohnDThompson

Other than the slight efficiency and wear advantages @AlmostTrick notes, if you do skid stops, a ring and/or sprocket with a prime number tooth count will give you the maximum possible number of skid patches, so 47/17 or 52/19 would be the preferred combinations of those you are considering.

Originally Posted by Mikefule

Small: slightly less weight. Slightly less frontal area for wind resistance. Slightly less chance of clonking the chain ring if you ride across badly broken ground. Slightly more wear, slightly less efficient.

Big: slightly heavier, slightly more wind resistance. Less wear, and slightly more efficient.

Most of us in every day riding would not notice the difference, but might make the choice on aesthetic grounds, or just on the basis of what's available.

Originally Posted by REDMASTA

I run 47t on of my bikes, keeps it simple.