I've ridden a lot on 26in bikes, commuting, med touring and heavy touring. From my experience, for light touring I prefer a 1.5, simply because as someone else brought up, there aren't a lot of narrower 26in tires out there, and the 38-40mm range is nice when going over rougher roads compared to 28, 32 or even 35mm, but there are so many variables of how heavy is bike, what sort of roads, what narrower 26 in tires are available, so a regular old 1.5 is an easy answer.
of course, putting on a lighter, more supple tire is always going to be nice for riding feel and better rolling, so a lighter slick 1.5 ish tire is going to feel and be a lot faster than a 1.5 super heavy duty stiff tire.
take a peek at my longish term review of the schwalbe supreme, I used 2inchers, but they come in a 1.6in , pretty impressive tires, but pricey, 50 bucks online, about 90 in stores.
https://www.bikeforums.net/touring/1...ly6000kms.html
perhaps give more details of your expected riding, bike, load, roads, etc