How big of a gear do you want to push? A 53 X 20 will get you 70 gear inches, but I have no idea if that'll work for your terrain. Personally, I'd use the 39T and fine tune it with a 14, 15, 16, or even 17T cog, depending on how hilly your routes are. Plus, you'll get better chainring clearance, if that matters to you.