Personally I'm happy with either.
I mean, it would be super stupid if it was just 'hey she's my childhood friend that means she's the right choice!'. he has had a relationship with f-cup and it seems to have gone well, they both seemingly care(unless another equally stupid development happens which undermines the entire setup of f-cup). even at this moment, watari KNOWS that f-cup really likes him, and knows that satsuki likes him but, and this is an important part, he doesnt know in what way. satsuki doesnt even know that herself. so honestly i find it realistic that he'd want to acknowledge that he does like satsuki, but wants to be in a relationship with the person he' enjoyed being in a relationship with so far.
but even if some switch up happens, i like both of them pretty much equally. they're both good for him in different ways.