Joined: Aug 2010
Posts: 12,728
Likes: 2,106
From: Madison, WI
Bikes: 1961 Ideor, 1966 Perfekt 3 Speed AB Hub, 1994 Bridgestone MB-6, 2006 Airnimal Joey, 2009 Thorn Sherpa, 2013 Thorn Nomad MkII, 2015 VO Pass Hunter, 2017 Lynskey Backroad, 2017 Raleigh Gran Prix, 1980s Bianchi Mixte on a trainer. Others are now gone.
Generally a 26X1.5 tire that is about 37 to 40mm wide is considered a narrow tire for 26 inch wheels. But a 700cX37 tire is considered to be pretty wide as 700c tires go. But, they are both about the same width.
Bottom line, if you are likely to stay narrower than 37 to 40mm, go with 700c. If you are likely to use a 37 to 40mm width tire as your narrowest, consider 26 inch.
I have several touring bikes, my 700c bike is my "pavement" bike, my 26 inch wheel bikes are my "off road" bikes. That said, I have used one of my 26 inch touring bikes with 40mm wide tires on a 500 mile on-pavement tour and it worked fine for that. Thus if I was going to pick one size for all purposes (on and off pavement), I would pick 26 inch. But if I was going to do all of my touring on pavement, I would consider both options.
This topic has been covered in great detail, do some searches for more info.