Depends on your barrel.. Well if it's an M4 barrel, than the Type B is best, but there might be some modifications needed as the description says on the page of the link.
Also, Type B is more secured, if you got the grooves on your barrel, and it will be already alligned straight. Type A uses nuts to secure onto the barrel.
If you don't want to modify your barrel when Type B doesn't fit, buy the Type A.
Type B will fit if you modify your barrel and make the grooves yourself.