Ethereum and Bitcoin are essentially two different technologies (protocols) within the cryptocurrency asset class.

Beware, you'll run into a lot of evangelists who claim one is better than the other. This is for you to decide. Most of the people involved are emotionally invested for one reason or another.

Ethereum is more of a platform. It is its own eco-system. It has a very flexible and continuously evolving programing language. This is why you see the majority of software development working with it. Bitcoin has been stagnant for years. The key to technology is to follow the development. No devs, no progress. I don't code but many developers claim that the Bitcoin protocol can be very difficult to develop on. Again, take this with a grain of salt and test things on your own if interested.

Ether is the 'coin' that is complementary to the Ethereum platform.

Bitcoin is what inspired Ethereum. Without it, none of this exists.

I hold both but every year I find less and less of a reason to support Bitcoin itself. Definitely read more into both and find what interests you most.