The question is wrong. I would ask "Is life with or without a relationship better"?

It's the relationship what makes the difference, not the marriage (at least it should not make a difference in your connection to your partner if it's your girl-/boyfriend or husband/wife).

The short answer is, "it depends".

Different types of people will give different (valid) answers.

People who love to be together with another one, where relationship is more important than being single, who cannot be alone, will be unhappy without a partner. So they seek a relationship to live a happy life.

On the other hand there are people who love to be independent. They want to do whatever they want whenever they want. A relationship can make this difficult.
A relationship is always a trade off of what you want and what your partner want. In the best case these two expectations matches, but usually there is a gap between those two expectations.
Someone who see less benefit in giving up some of his freedom than the benefit of being in a relationship would sooner or later be unhappy with the relationship.

And there are also people who don't see any benefit to be in a partnership at all. For them a relationship would just be a pain in the ass, so they stay single and be happy with this decision.

Everybody in this range from "cannot live without a partner" to "cannot live with a partner" has his valid reasons and they should be respected.