cogs are generally cheaper than chainrings, and a 16T cog is plenty secure in that situation.

it also depends on the BCD on your cranks. if it's 130, you don't have many choices for single speed rings, although TA makes a few different sizes. basically, if you want to save money, get a slightly smaller cog, as it won't really have much of an effect until you get down to like, 11 or 12T cogs.