I'd recommend going with CM16's, simply because they already have an RIS system, so if you choose to put some attachments on it (which I am almost certain you will in the future), you won't have to spend additional money to get an RIS on the King Arms M4A1.
Also, G&G's build quality and internals are quite good for a beginner gun, and might be able to even compete with some of the higher builds.
P.S. I would remove the link to the store, as it is not allowed to post that outside of AV section. Just link pics instead.