If I already watched Cosmos 2014 but haven't seen the original...

Degrasse's series is actually a bit more informative in terms of history and facts regarding science. His series teaches the philosophy of science through historical example.

Sagan's series does some of the same thing, but it's slightly more narrative in nature because Sagan's voice explains what Degrasse's series does through visuals.

The series with Sagan, which I saw when it first aired, is a bit more primitive in the special effects department. You'll get a good sense of what science and computer technology was like in the late 1970s. For that you might want to watch it.

But you're not going to get too much new factual science information from it, but each series covers science history that the other doesn't.

I hope that helps.

Do yourself a favor and watch the original. It is vastly superior to the new version, which contains too much climate change nonsense and political garbage.

Schrodinger's cat walks into a bar, and / or doesn't.

"If I already watched Cosmos 2014 with Degrasse Tyson, would it be redundant to watch Cosmos 1980 with Sagan?"

No, it wouldn't be redundant to watch the original version. The original and new version of Cosmos are equally informative on the science they present.

I've got the box set of the original Cosmos on DVD. Some of the episodes include an update of scientific developments and discoveries that have happened in the 10 years since the episode was shown. For instance, there are episodes in which Carl Sagan warned of the dangers of nuclear war and global warming. The updates are post Cold War, in which Sagan applauds the wisdom of nations moving in the direction of nuclear disarmament, but still warns of the dangers of global warming. Here is how Sagan expresses it:

https://www.youtube.com/watch?v=G70lxDMRO3Y

At the very least search out the original Cosmos soundtrack album/CD “The Music of Cosmos”. It’s fabulous.