I've enjoyed reading the Alexis versus Erynn debates so much I've decided to throw in my two cents.
I tend to play as if I'm the MC. When it comes to Alexis versus Erynn, I ask myself with whom am I likely to have a more fulfilling long-term relationship based on their levels of empathy, attraction and commitment, and "brains" (though not necessarily academic intelligence). In my view, the answer to this question is Alexis without a doubt.
1. Consider the reactions of Alexis and Erynn in the flashback scene when the MC confesses to Erynn. Erynn didn't laugh at the MC, but she didn't immediately go after him to comfort him either, despite the obvious emotionally fraught nature of the act of confession and the fact that they were supposed to be such good friends. Alexis comforted the MC immediately even though they weren't really friends. Empathy advantage goes to Alexis.
2. The sexual attraction between Alexis and the MC is off the charts going both ways (if you as the MC elect to proceed with Alexis). Whether or not you believe in love at first sight, I've found through hard-won experience that a relationship with a strong *mutual* sexual attraction has a better chance of long-term happiness than a relationship in which one partner is more attracted and/or committed to the relationship than the other. As others have pointed out, Alexis repeatedly makes decisions that are adverse to her other interests to be with the MC. Erynn doesn't do that at all. She didn't try to visit him after he left or otherwise stay in touch, and it appears Erynn hadn't made it known to her mom or the MC's mom that Erynn was really into the MC (otherwise, why didn't one of the mothers tell Erynn when MC came back to town?). It appears that Erynn's confessions to Alexis about how much Erynn misses and/or wants the MC might have had more to do with her feelings about the loss of his friendship than Erynn being powerfully attracted to him (i.e., is she confusing love and friendship?). The advantages of mutual attraction and willingness to sacrifice other goals for the relationship go to Alexis.
3. The apparent elephant in the room--the fact that Alexis pursued the MC despite her friendship with Erynn--doesn't overcome Erynn's lack of empathy and lack of strong attraction to the MC. There is a big difference in cheating on a lover while in a committed relationship and pursuing a partner who even your "best" friend has an interest in, especially on the facts here. First, it appears obvious that Erynn just isn't into the MC as much as Alexis is. Second, were Lexi and Erynn best friends forever or, like many friendships formed in childhood, friends of convenience because that's where they grew up and, in this case, because their parents were great friends. I'm no longer close friends with *any* of my friends from my childhood. I'd willingly sacrifice one of my childhood "best" friends if I thought I'd found someone who could be the love of my life, and I didn't hold it against Alexis that she was willing to do that for the MC. Friends often come and go throughout one's life; the love of your life is forever. No advantage to Erynn on this one, at least from my perspective.
4. Alexis's lie-by-omission of not telling the MC about her friendship with Erynn is a harder call, but I still found myself forgiving her relatively easily. It's common to omit a lot of potentially unpleasant details when a relationship first starts. Why lay dirty laundry on the table if it's only going to last one or two dates anyway? While this is a strike against Alexis, it's not surprising under the circumstances and doesn't rise to the level of a break-up offense. In any event, this issue doesn't give Erynn an advantage. Early on, Erynn also lies-by-omission to the MC by not disclosing the nature of her ongoing relationship with Ethan and the truth about her "modeling" job. There is no relative advantage to either one on this point.
5. Finally, Erynn is too foolish to be a long-term option for me. Her "modeling" career is a joke, and her inability to find some alternative course of action for dealing with Ethan's blackmail over such a long period of time is astonishing (at least one or two years had already passed since the video was recorded). Was her plan to just keep doing what she's been doing forever until Ethan turns her into a literal whore? It appears so. I'm just not into the dumb, helpless blonde trope. Alexis doesn't appear to be a rocket scientist, but the brains and maturity advantages still go to Alexis.
But hey, that's just me. I like some of the other girls better than either one of them. Team Nikki anyone?