I'm curious why you feel a need to determine which is worse.
Take my and my H's affairs as an example. They were a tangled, madhattering shit show back before we were married. He cheated on me first. He had a teenage gropefest for about 15 minutes with my SIL's BFF. He'd had a crush on her when he was younger, and when drunken opportunity knocked, he answered the door. He tried to confess the next time he saw me, but when he saw the panic rising in my eyes, he chickened out and said he'd only thought about it. He then let the lie stand for the next two years. During that time, he suggested that we could be open to seeing other people, hoping that if I took him up on it, it would soften the blow when he eventually came clean. I was brokenhearted and thought it meant he had one foot out the door.
About a year after his fling (and still in the dark about it), I became attracted to the guy who lived across the hall from me in college. I wrestled with this for a few months and then decided to tell BF that I'd like to take him up on his offer and date the new guy too. BF said it was fine as long as it stayed casual. We didn't specifically define what that meant, but it was pretty clear that it was PG-13 stuff; BF and I were first-and-onlies, both virgins when we got together. Over the next four months, while BF was working long distance saving up for our future, I got progressively more involved with the OM, and I ended up sleeping with him. I broke it off and told BF about the sex and "I love yous" the next time I saw him. However, I minimized the full extent of the EA/PA, in which OM and I were inseparable for the last two months.
BF was destroyed. He demanded that I go NC with OM. I refused, insisting we could be just friends. BF also came clean about his fling but considered it irrelevant in light of what I had done. He decided that he needed to get his own back by having an ONS with a girl he knew (telling me his intentions in advance). They got to the naked stage before he bailed because the whole thing felt gross and predatory.
I could go on and on about the unhealthy mess we made of our relationship, but the eventual outcome was that I went fully NC with OM, and BF and I got engaged. I let my lies stand for almost 30 years until BF (now BH) had a midlife crisis and asked for more detailed answers. I panicked and trickle truthed him for weeks before finally coming clean.
So who was worse? Well, he cheated first. He didn't ask about dating OW beforehand, and he didn't tell the truth afterwards. If he had, I might have been a free agent when I met OM. I was honest about starting to date OM and honest about the sex afterwards. However, my affair was far more extensive, and I lied for far longer. He had two APs, making him both the first and the last person in our relationship to be physically involved with someone else. However, he didn't have an EA with either one of them, while I fell for OM.
I have my opinion of who was worse, and so does he. Our opinions match, as it happens. But what is the value of comparing? Does arriving at an answer make either or both partners more willing to do the work? Does it help you heal? Or does it drop both of you into a warm bath of victimization and self-justification, where you deflect your attention to each other's transgressions instead of diving into your own?
When you focus on who was worse, it distracts you from how to become better. So again, I'm curious: why do you want to know?