I always consider the number of skid patches in choosing gearing. Another factor to consider is the even versus odd number of teeth. Sheldon Brown says you can extend the life of your chain and cog combination by using an even number of teeth on both the cog and chainring.

http://sheldonbrown.com/chain-life.html

The idea is that each cog tooth interfaces with only chain links in the outer plate or inner plate section of a link. I believe Sheldon is correct, but I choose to use an odd number of teeth for either the cog or the chainring anyway. My reason is that I know I am not diligent enough to maintain a consistent phase relationship of the cog and chain during maintenance activities. I prefer even wear on all the cog teeth at the expense of shorter cog life.