Some differences to note:
1. Novara come with bar end shifters, and these can be fitted on thumbies (see Paul's thumbies) on the bar you plan to get so you would not have to change shifters.
2. Novara has better crank gearing for touring in hilly terrain (48/36/26).
3. All come with lower end hubs, but the KHS comes with 36 spoke wheels, the others with 32. A bit stronger wheel on the KHS. Check spoken tension before riding - likely these wheels are machine built.
4. REI has the best warranty of the three - best of most any bike company out there.
5. REI credit card offers 5% return for items purchased at REI, so get their card and save 5%. In addition, get $100 gift card after first purchase. (
REI MasterCard® - Card Benefits )
6. Novara is 10 speed, others are 9 speed. Read about the differences between these - strength, maintenance, replacement, etc. I would prefer 9 speed (or even 8 speed).
7. Marin comes with cassette with lowest gear of 32, other two 34. Nice to have a bit lower, 34, in steep climbs.
8. The rack and fenders on the KHS will work, but look low end.
9. Marin comes with Schwable tires, Novara with Clement, KHS with Kenda. No double many will like Kenda, but I've had poor experiences with Kenda tires. Schwable makes great tires. I don't know the Clement brand.
10. Brakes - Novara come with TSP Spyre which gets good reviews. The others come with Promax or Bengal disc brakes. I don't know much about those.
Review: TRP Spyre SLC Mechanical Disc Brakes | road.cc
11. I think all come with brakes that require short pull levers, which means when you replace your handlebar, you may need old-school mtb brake levers with short pull, or one that can switch between short and long pull. Most new mtb brake levers are long pull. Possible you will need to ebay old-school canti-brake levers - some nice one's out there for cheap now.
12. Cannot find specifications, but it looks like the Novara comes with the most tire clearance in case you wanted wider tires.
13. Novara and Marin use hollowtech, external bottom brackets, KHS uses octalink. I would prefer the hollowtech version.
I would probably go with the REI Novara.