One of the nice features of working with a biscuit joiner is that all your joinery needs can be met by a very limited number of biscuit sizes. If you keep a supply of the two or three biscuit sizes commonly available, you will be good to go on any project.
The rule of thumb in biscuit joinery is to use the biggest biscuit you can; this will give your joint as much strength and stability as possible. The only real question is whether any particular type of biscuit is more suited to your project than another. The short answer is “no.”
Wooden biscuits are made in two variations: solid beech wood (the original and most readily available) and plywood, in which very thin layers of wood are used. For each, the wood is compressed so that when exposed to liquid wood glue it will expand and form a strong joint. Since the solid beech biscuit was actually invented for use with plywood as well as with solid planks, there is no compelling reason to seek out plywood biscuits for your plywood project.
There is a newer product made of plastic that uses the design of the biscuit to secure the joint. It is faster to use since there is no drying time, but it is relatively expensive and hard to find. Also, the joints it makes are visible, so it lends itself more to production work than fine joinery.