I am installing 12"x12" marble tile in my shower. I was told to use a 1/4" notch towel and 1/8" spacers. I am confused as to the choice of thin-set I should use for this job. Also I wonder if 1/8" spacer for a non-sand grout will be the best choice for the job. Your advice is appreciated.

Thank you,

The premixed thinset generally contains latex. I like to use it for wall tiles because I think it adheres the tiles better. For a shower it will be fine for the floor as well. Just make sure you put a solid layer of cement board down on the floor and even if the premixed thinset says you can use it for grout don't because it never gets hard like traditional grout does, it stays flexible almost like caulk. Non sanded grout should be fine however I recommend you make a mock up on board and experiment with two tiles and see what you like best.

I'd use the smallest spacer I could find. Marble looks best with smallest grout line possible. Use premixed thinset. With a small area like a shower its not worth the cost savings to go to the trouble of mixing.