I don't think one is inherently better than the other. It all depends on the person, what they value, who they're with, where they are in life, what they want in a relationship, etc etc.
I'm polyamorous, but I've been in both monogamous and polyamorous relationships. I'll never forget the moment I realized I didn't care for monogamy, personally... I was just 18 and my ex called me (we were dating at the time) and confessed to me in a very intense and heartfelt manner: "I made out with someone else when I didn't mean to." In tears and truly apologetic, I'm sure my ex thought that it would be a dealbreaker and I'd be very disappointed. Now, I loved my ex. I asked "Do you still love me?" to which my ex responded, "yes, of course I do." And really, that's all that was important to me. I didn't mind if my ex went and made out with another person if, at the end of the day, we still loved each other. I even asked, with genuine curiosity, if it felt nice. I really hoped it did, truly! If my ex making out with someone made them happy, then why would I want to stop that from happening?
Since then, we've broken up for unrelated reasons (we both wanted to live different lives in different countries), but that moment was one that really made me think about relationships and how they are formed. When we dated, we went in thinking "ok, we are dating," but never really ironed out the details of our relationships. We never really asked "what things are ok to do with other people?" so I never explored the thought that I was ok with other people being involved.
After that incident and reflection as to why I felt how I felt, I believe that my relationships should have a "contract" that clearly defines what is expected and what the boundaries are. And in general, I think this would be good practice to other people as well.
Why? Well, monogamy and polyamory are looked at in a very black and white manner, but I think there's more bleed than what meets the eye. I can't count how many straight women I know who are in monogamous relationships with men, for example, who allow them to "sleep with other women, but NOT with other men." Sounds awfully polyamorous to me, but they consider themselves completely monogamous. Of course, there are varying degrees to polyamory too! Some people simply form noncommittal bonds with others while others truly want to live with more than one spouse as a family.
People should have the ability to know exactly what their relationship is and how it should be navigated (and of course be able to change things with effective communication if needed). That, to me, is one of the biggest keys to success in making a relationship last- monogamous or polyamorous. Either is possible.
Fast forward to now. I've been with my lovely s/o for nearly 3 years now. In the beginning, before we started dating, I said: "If we are to be in a relationship, I would need it to be open. That's just who I am and what I prefer. We can iron out the details, but generally that's what I want. Now, I want you to reflect on this and if it aligns with your values and what you are looking for, and let me know when you are ready. Do not give me an answer at this moment when we are next to each other, because I want you to truly think about it and pick what is best for you."
After nearly a week, my s/o agreed and wanted to enter an open relationship with me. We made our relationship completely open to others. In other words, we could form romantic and sexual bonds with others how we saw fit but we weren't necessarily looking to add a "third." There have been some people I've been on one date with, and there have been people I've formed years-long connections with and cherish deeply. We even introduced and shared our newfound connections with each other sometimes, which was always nice. Spreading the love and whatnot. And every single day through our relationship, I've continuously loved, supported, and grown with my s/o. We are truly happy together, and we have so much love in our hearts for each other.
so, tldr: polyamory is my personal preference but I think neither is better than the other. An important thing is communication between those involved to truly define what is being established, needed, and unacceptable. Either can work.
