The new Sig Classic series has push out takedown pins front and rear like the AR.
I have both an AR-15 and a SIG 556. I trust my AR completely as I don't have a lot of time with the SIG, but I've had no malfunctions with either and I like both platforms for different reasons - the AR for familiarity and the 556 for design and reliability.
If this is your first black rifle, I'd almost recommend you get the 6920. But the 556 might be harder to get than an AR if something happens politically and people start panic buying like crazy again.
The Colt will always hold value.