Sam's Diet Cola vs Coke

So I've been getting the Walmart version of Coke, just cause it's usually $0.58 instead of $0.99 (on sale, $1.50 otherwise). I've decided that it isn't the taste that really is the difference, but the ability of the soda to remain carbonated. I tend to leave my cups of soda for a long time, so I notice the non-fizziness. After a few hours, the fake Coke is flat and tastes funky, the real Coke (or in this case Coke Zero) still has some fizz and tastes okay.