Obviously a happy ending is really nice. I was reluctant to accept the sad ending (if it happens) at first but now that i give it some thought, i think it's actually better. Firstly if they do end up marrying each other its just another happy fairytale. Second, if they do separate it makes the story more realistic and more romantic.
They were each other's no. 1s but sometimes in life you gotta settle for the number 2. If they separated, it means they love each other enough to respect each other's decision and opinions. They are also mature enough to know that this is the best choice to make. Instead of dragging each other down from achieving their dreams and resenting each other, choosing to let go and love each other as a memory is more romantic.
If one if them gives up their dream so that they can stay together, I'm sure they will feel a lot of regret.
In the end they will be someone unforgettable on each other's minds.