@lg Thanks for reaching out!
You are correct that the difference between these two devices is the preloaded topo maps. The 66st comes with 100k topo maps loaded on to it. You can still add microSD card to the 66st. While REI does not sell Garmin 100k topo maps, they are available on Garmin's website for sale and download. REI does carry 24k Garmin Topo maps on microSD cards.
Hope this helps!