Its hard to recomend a good ratio with out knowing a few factors: how fit are you(stamina and leg muscles), how long is your commute, how often will you ride, what are you goals, how hilly is your ride, do you use brakes, is this your only bike, etc.

Generally if you find it to hard to start, your muscles tire easily, climbing hills is too hard or stopping is to dificulat you should gear down(smaller chainring, bigger cog)

And if you spin your pedals too fast, your stamina is drained to quickly or you cannot reach a decent top speed you should gear up(bigger chainring, smaller cog)

Chaining cogs is cheaper, but chainrings come in more sizes.

A few thing to think about, chaining the cog changes the ratio more dramaticaly. Fixed cogs can only be easily found up to 22 teeth, freewheels 24. For the cranks most people have(130 bcd) the minimum size you can get is 38 teeth. The biggest I have seen is 54.