As others have mentioned, you do not indicate what is important to you, i.e. display screen size (not that important while touring as I can always stop if needed), accessories like heart rate or cadence, etc. For me, the unit’s capacity of way points, can use standard-sized AA rechargeable batteries, and battery life are the most important. I currently use a 64s.
I have toured for over 40 years. At this point, I create my own routes using waypoints (not track points) and I have over a thousand little connector routes (most under 50 miles) covering North America. ALL my routes are created prior to actually riding. Think of them like a car’s broken windshield, lots of little lines interconnecting. When I want to go on a tour, I download the various routes together along with the corresponding “service points” (waypoints I added that are verified locations of grocery stores, campgrounds, POIs, etc.) averaging about 40 Directional Waypoints and 50 Service Waypoints per route.
Garmin typically limits routes to 50 waypoints or less for turn by turn directions. A unit’s capacity is limited as to how many waypoints, or track points, it can handle. Most units handle 200 routes but the overall capacity of waypoints ranges from, I think, 1,000 to 6,000 depending on the unit. I think most handle 10,000 track points which sound like a lot but it really isn’t for a long tour. Regardless of the external memory capacity, the waypoint/route/trackpoint limit stays the same unfortunately.
Since I tend to tour small highways/back roads with lots of turns, I may bring 40 routes on a tour or up to 4,000 waypoints. I have once run out of waypoint capacity on a unit but it was an older unit. My tours tend to be 6 weeks or less for now but anything longer or if east of the Mississippi (more turns), the capacity would run out pretty soon.
Sooo, if you do custom routes, I would focus on waypoint/track point capacity. If you just want to follow an ACA route, do not get a GPS but use your phone or tablet and download their new route apps for $4.99 each and use your phone to search for grocery stores, etc. when you need them. I “think” the apps can be used off-line and are really nice if you can just follow a line.
For me, the screen size is not a huge deal. I have a Montana also which has a nice touch screen display but it is a battery hog, about 3 rechargeable batteries per day. That is a lot of batteries to keep charged or buy and carry. For me, the 64s uses about 1.5 rechargeable batteries per day on average. My old eTrex 30 (circa 2005??) would go 3-4 days on 2 batteries.
All that said, in reality, if you have an accurate bicycle computer and you just pay attention to it and the maps, you do not need a GPS. Yes, they GPS are convenient but at times they are a pain with battery management, become dependent on it, etc.
Best, John