Int comparisons are faster than varchar comparisons, and ints take much less space than varchars.
This is applicable true for both unindexed and indexed access. You can use an indexed int column to make it faster.
If you are interested in the space usage of different data types:
int fields always occupy between 2 and 8 bytes, with 4 being usually more than enough ( -2147483648 to +2147483647 )
character types occupy 4 bytes plus the actual strings.