I'm getting ready to tile a small bathroom floor. Is the premixed thin set as good as mixing the powder up? Thanks

The premixed is the definite easier way to go (and just as good outside of attaching fixtures such as soapdishes or shelves on the wall), if it's a small bathroom it won't cost that much more but it will be well worth it to you. Mixing mortar is a messy job and it sets up A lot faster than the premixed.

Make sure its thinset and not mastic premixed, the mastic want work with large tile, only 4 and 6 " the larger tiles don't allow enough air to dry the mastic.
Id use thinset that you mix, but that's me.

Main thing is to make sure its not mastic that your buying, either thinset will be fine

Thanks!