I've also got both "Sporter M4 Carbine" and "M4 Carbine" marked 6920 same as you but both mine are shooters, I'm a very long time Colt guy for 41 years now and I shoot all my Colt, sell them when worn to buy new Colt models.
It seems the Sporter had a lower quantity made, mine is an early production and comparing that with the newest one I had seen there might only be 20+K made while my M4 Carbine is also an early production, compared to the latest ones now seems they made about 70+K already.
I thought the factory had stopped making the Sporter version late last year but there was a member that just got a new one made early this year, maybe they're making limited runs depending which distributors orders them with enough minimum quantities.
If I have to keep one NIB myself for collection I would pick the Sporter because they made less of them.