I have to say I have very similar thoughts when it comes to Maya, in that she reminds me of one of the first women I was serious about/with. Which is why I'm inclined to chose sage. Don't get me wrong I look back on those memories fondly, mostly. Where as Sage doesn't drag me back to bitter sweet memories.
It's a bit bittersweet with Maya for me too, but she's also type of girl I go for and there have been a few since then, so it's not that bad looking back. With Sage though, I feel like the MC has caught her at the wrong time. She's been besotted (for some reason) with this meathead Jock, Chad, who doesn't seem to have really given her much attention and affection, and now she's broken up with him and the whole thing has devastated her. If the MC and her get together now, then he's just a rebound and she's not mentally or emotionally ready for another relationship right now, so it's probably not going to work.
This is why, with Sage, I'd hope to be able to just keep to FWB because I see her as a really good friend to the MC who is there to help Sage feel better about herself and to open up more so she can find someone who will treat her right.