I think a 20T cog is "huge"- the 39 chain ring is fine if you have the right cog, although it appears some people might be a bit insecure about running a smaller ring in front, but since you already have a 42, I'd put it on.

A 39X14 actually gives a plenty steep ratio (steeper than the 46X17), but a 42X 16 might fit all occasions.

Frankly, a nice setup is to have both a 14 and 16 and a 39 and 42 and mix and match according to mood.