I'm not sure if you're getting Mapei premixed thinset or in the bag. Only reason I ask is because the box stores around here only sell Mapei as premix. As an alternative, I've had great results with Laticrete's 317 Floor-n-wall thinset which you have to mix on your own.
In the Mapei line, I really like their Keracolor series. Doesn't turn into a concrete block on a DIY'er immediately.
Good luck on your project.