If two gear combinations (chain ring A and cog A vs chain ring B and cog B) produce the same number of gear inches, the gear combination that has the greatest number of teeth will have a tiny, theoretical advantage in efficiency.

The gear combination with the greatest number of teeth (with chain) will also weigh more, everything else equal.

Personally, I pick gear combinations with a prime number cog and an end result of 63 gear inches in mind.

For me, 45 X 19 with 28mm tires gives me my ideal of 63 gear inches:

http://software.bareknucklebrigade.c...it.applet.html