I have simply defaulted to the most forgiving thinset on every tile job. I skip the $8.00 bag and the $12.00 bag and the $16.00 bag and go right to the $24.00 bag. It costs more money sure, but its tiny in the scope of most of my work and I know I will have the least amount of problems using the best thinset available.
you need to give more info. The cheap stuff is unmodified and is really only good for under some backerboards, not all manufacturers approve of this. You can use it to tile over concrete but its not as forgiving of poor techniques.
The midrange stuff is modified with laytex and will do a good job for most basic tiling jobs if the floor is fairly flat. The high end stuff is designed for heavier tiles like Marble. Its extra ommph keeps the tiles from sagging down unevenly in the setting bed so you don't end up stubbing your toe. The high end stuff is also good if the floor is uneven and you want to use a bigger notch trowel to help flatten out the finished floor; but you're much better off addressing an uneven floor before you start tiling.
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