Despite warnings by some experts, I went with YHM on my 20" upper. I'm very impressed so far. I do not have the Diamond Series, but do have their lightweight forearm like thousand others. YHM makes a bipod that screws on the front and they sell a disk that covers the hole up front.
I vote for the Diamond Series, and I may get one for my 16 inch Carbine, when my fund manager, aka wife, approves the purchase.

Just my biased opinion. ("What I have is good. What you have is crap.", logic)