Hi Mark
I would do the walls first....leaving out the last row at the floor.(you'll need to put up a straight edge here anyway to support the tile as you work your way up...just make a level line and nail on a straight piece of wood along the line)..just don't put it so low that the nails puncture the liner. After the walls are done, you can do the floor,and then fill in the last row(s) of wall tile. The last row will also cover the joint against the wall. Either one of those thin sets will work fine....but if you are grouting white,or another light colour, it will be a good idea to use White Thin set....it makes the clean up a lot easier. Any spots you miss using grey thin set will show up against the light grout....white thin set seems to blend in.
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Brian