I'm hanging 3 x 6 x 1/4" tile in a half shower. I've got durock on the
bottom foot and treated sheetrock on everything above.

So I can tile the bottom of it conventionally, and would expect to use
what they recommend at lowe's: the tile mortar with polymer.

q1) There's two of them, one for $18 and another for $24. What should
I look for? By no means do I adhere to the Law of It's better because
It Costs More.

I wish I hadn't hung treated, but I did. I know I can make tiles stick
with all-purpose construction adhesive. But I'm in the market for
something that bridges that gap between the two.

"Mastic" doesn't really exist anymore, at least at the place where I'm
constrained to buy things that I can. That said, to find the right
adhesives or additives, I will seek out any bona fide recommendation.

Things are good in my hood,
--
Uno