i change the chainring. 48/19 and 42/17 are both about 65 gear inches, what i like to ride in the city. with the 42 tooth chainring, when i feel like riding the freewheel side i only have a 1 tooth difference (16t freewheel) as opposed to a 3 tooth difference(about 10 gear inch difference) if i just change the cog.

so, getting a $40 chainring is cheaper and more versatile for me than buying a $30 cog and a $25 freewheel.