When you're choosing a thinset mortar, the composition will depend on where you're going to put the mortar and what the particular application will be. There are two different types of thinset mortars: polymer-modified and dry-set. Dry-set mortars can be mixed with water or with polymer additives if you need a greater bonding strength. Polymer-modified mortars already have the strength added and just need to be mixed with water.
Consider the places where you're going to use the thinset mortar. If you're just tiling on a bathroom or kitchen wall or making a decorative project, you can use dry-set mortar and just mix it with water. The same goes for tiling on a cement substrate. However, if you're using fully vitrified porcelain tiles, you'll want to use a thinset modified with latex or dry-set with an acrylic latex additive.
Choose polymer additives or polymer-modified thinset mortar if you have to meet ANSI requirements A118.4 or A118.11. You must have polymers in your mortar in building projects that fall under those guidelines.
Go for the polymer additives or polymer-modified mortar if you are using it in one of these specific situations: to cover exterior-grade plywood; to cover plastic laminate or ceramic tile; to cover plaster or drywall with paint on it; to seal any areas that will be exposed to freezing temperatures, or to cover cutback adhesives. For best results with vinyl flooring or on a wood base, you will want to consider latex modified thinset mortar, which you'll usually find labled as Multi Flex, Super Flex or Full Flex.
If your grout will be light, choose a white mortar. If your grout will be dark, choose a grey mortar.