Autotiles have no 4 directional passabilities. At all. THey're either passable or they are not. A5 tiles are NOT atuotiles (they are fixed tiles) and can have 4 directional passability.
For mapping cliffs, simply put: if you use an autotile, all those graphical rock borders mean jack **** to the program. It would be the same as mapping in full raw grass. All the editor knows is that it's "passable". It has no borders as far as it's concerned.
A5 tiles can be configured, and the editor reads them when calculating passability.
"And Indrah, the walls can do one thing the floors can not: 1-tile-wide branches of cliff, or however you want to call it. You always need two floor tiles to form a branch. Or tongues. I don't know the proper term."
I have absolutely no idea of what you mean. THe cliff walls in A5 are duplicates of the autotile, anamely the second and third tile rows in a 3*3 setting.
Bascially you can do the same damn thing with the cliff wall autotile and shift clicking, and you probably will anyway, since the A5 section does not have the "first row" tiles (the horzontal tiles closer to the upper cliff end, so you'd have to shift click THOSE).
Pff, Frick this. Have it visually. It's hard to convey in text.
Bottom line:
They're the same damn thing. In fact, using them is more work than simply shift-makkign with the auotile in whatever size you need.