I'm inclined to give David Horowitz broad leeway on some things, but this is simply not an area of his expertise. His parents were Communists. That doesn't mean he knows anything about the extent of penetration into FDR's administration. As a matter of fact, if anything he would probably know less because Red Cells were highly compartmentalized as a matter of tradecraft. So his involvement in this dispute is puzzling.
Radosh is another story. I personally think Radosh has played the role of limiting the breadth and depth of revelations about American Communism and Soviet penetration. His narrative about John Stewart Service and the fall of China is simply not to be believed. I'm also skeptical about his book about the Rosenbergs. Ann Coulter slapped him down very hard after he attacked her for defending McCarthy. From what I've read about McCarthy from a number of sources, Radosh was carrying liberal water, and Coulter was right about him. Personally, I don't think Radosh is trustworthy.
But I want to read and hear more comments by other conservatives before I make a judgment about West's book. Again, I read Evans book about McCarthy, and I liked it a lot. However, let's see what other big name conservatives say about West's book.