What is the diffrence between Phenolic and Micarta if any, and which is best to build a riser from?

Phenolic is usually a wood based laminate like paper.  Micarta can be laminated out of a variety of cloths, linen based micarta is very popular.  

I've used both quite a bit in my industry but not on bows.  In my experience micarta is a bit more durable and tough but, both are more durable than wood and either should work fine in a riser.

Micarta is a trade name from one company and can be paper, linen, or other.  Phenolic is the generic name and can be different materials as well.

I have zero experience building a riser from the stuff, but I've only seen linen used.

I put phenolic strips in all my risers. It is tough and strenthens the riser a lot. I am using 1 strip 1/4" and 2 strips 1/8" in my next riser. Don