In general, it's cheaper to replace cogs than chainrings, but cogs make a more drastic change in the gearing. As already noted, going from 16 to 18 is a pretty big jump. You might try a 17.
As an avid singlespeeder, I have amassed a pretty large collection of thread-on freewheels, individual cassette cogs, and chainrings over the years. I consult
gear inch charts and try to put all of my singlespeeds within the same range -- I like the low 60s for the kind of riding I do, which includes some fairly stiff climbs. As I've aged, I've gone to easier ratios -- my glory days of 74 gear inches are well behind me!
Point is, it's all about experimenting and finding what works for your fitness and terrain. Cogs that you buy and don't use can be installed on future projects; there are lots of combinations that yield similar gear inches. It's all part of the fun, IMO.