Which is better, the monastic life or marriage?

It depends on the person, as Paul says in 1 Corinthians 7. Paul wished all could be like him, unmarried, so they could be free of preoccupation with spouse and children to help get the gospel out, but he acknowledged God gives one man one gift and another man another. We are made sexual beings, made to be attracted to the opposite sex, and that is how the human race is propagated, so I would say the monastic life is actually the harder choice, because you have to resist the temptations of lust every single day, and it never goes away. Marriage looks like the easier choice, but once you are a few years in, it becomes the harder one, because you must be on your best behavior every day for the spouse you live with, watching your moods, your words, your spending, with no excuses, for the rest of your life, since Jesus said marriage is for life and you cannot remarry except in the case of fornication. Yet the monastic or religious life, filled with searching for a God whose depths you will never plumb, is a joyous occasion, and I have never met a nun who said she made the wrong decision, whereas I have seen many married people who wished otherwise, and many divorces. So it is not a matter of which is objectively better; both are good, and it depends on what God has given you and what best fits what you know about yourself.