I'm not sure how to find out what kind of hair a doll has before buying it (Google?), but if you cut little pieces of hair you can find out by burning it and/or dunking it in water. The different types of hair will react differently. I tried it once, it's fun

I'm not sure about the difference in quality, I do know that saran hair is favourite for most but I don't really know why..
I do know that I prefer saran hair for (re)rooting my dolls. Nylon and polypropylene are a close second and third. Kanekalon is my least favourite to work with, I used it once and gave up very fast, now I just don't buy it anymore.
If you're not rerooting and you want to know the hair type for styling your doll's hair, you should know there are different advised temperatures to expose the different types of hair to. For instance, on type - not sure which - only holds a style if you boil it, the other can't handle boiling and will hold a style at a much lower temperature.. if this is the reason you're interested in the different types of hair, I suggest you do a google search because I don't know the details from the top of my head.
But my question to you is, why do you want to know? ^^ This will help when trying to answer your question
