I've only ever handled a Sig 556... it felt solid, manual of arms similar to an AK, but with better ergonomics and controls placement. The early QC issues seem to be taken care of.
I used to own a Benelli M1-S90, which is the predecessor to the M4... the main difference being the lack of gas-assisted operation on the M1. The M1 was nice and light, reliable, fast, and accurate. I didn't have a need for it, so I got rid of it. But if I ever had to fight with a shotgun, I wouldn't have any qualms with a Benelli.
And I have handled and shot it several times. It's an automatic shotgun and has a hard kick which is surprising because my Mossberg with the same shell doesn't kick as much. Never had any jams or problems with it and it's a solid shotgun.