I just went ahead and tested it; look's like I'm wrong. It isn't the same.
I was getting hit for 20's by the dragonfire when using a pot alone. I know with a shield it's roughly ~5-10 but I didn't test a shield alone. Regardless, you don't get hit by 20s with a shield on.
I'm going to guess it's like 70% protection from shield and like 30% protection from the potion.