The difference is... there is no Zoey path if you are in a Maingirl relationship. MC rejects her automatically, at least i didnt saw it otherwise... maybe with enough cheatpoints, but i dont think so, cause going back together with Zoey is nothing like cheating, its starting a relationship based on love. Wouldnt make sense as a cheating fling.
But iam sure there will be an angle where MC can breakup with a maingirl and go back to Zoey in the future, but only if the relationship will be unstable i think... so you need some choices that makes said relationship unstable to get back to Zoey, otherwise, not happening...
I dont see a point in starting a relationship playthrough and than to decide to go back to Zoey anyways. Its clearly a better suited way for the others route. And just because of the sake of even more drama? Ah come on, there is enough already... if you wanna see girls cry over the MC, play an all maingirl chick route and see what happens...