Okay, so I have completed A New Order quest and have now reunited back with Serana. She is a good character and all, but I absolutely hate followers in this game, because they're so buggy and never follow you correctly. Will I be missing anything for not having her follow me around story wise?

Also, is it me, or is the Dawnguard Armor (Heavy) not so good? I smithed it until it was fully Legendary. I have a Smithing skill of 100, I even wore some items and drank a potion that improves smithing. I got to 180 points of defense and compared to my Daedric armor which is 300, it isn't so good. A giant actually causes a lot of damage while I am wearing Dawnguard Armor, nearly got killed, that has never happen while wearing Daedric. Am I doing something wrong when it comes to improving the armor?

It's Dawnguard. That's Daedric. Seems straightforward. One requires a heart of a Daedra. The other was put together with leather and metal.

And to answer your first question, I'd keep her around for the whole DLC. She has a lot of interesting dialogue for every part of Dawnguards main quest as well as optional dialogue too.