Surprisingly, I have to say I prefer A. ^^; As a few people have said, when 'will-they-won't-they' is done right it can be AMAZING. A series-long exploration of the characters that actually develops them and has them interact and grow together, questioning their choices and ideas, until they finally overcome the insecurities, misconceptions, or societal pressures keeping them apart?? Probably one of the most satisfying types of stories out there. 
Unfortunately, as more than a few people have said...it's only rarely done right. ^^; One misstep that I particularly dislike is when they only use the story to develop one side of the romance; usually the 'bad boy' or 'rough around the edges' partner who has deeper issues...and meanwhile, the other partner is reduced to a mere object of affection, whose moments of falling in love (if they even get any...) are made up of characterless gawking and blushing. =/ Or those generic compliments like "you're the strongest person I know" "you're not afraid to be yourself" blah blah blah...
Now, I really wanted to choose B...it sounds like an idea with more depth, and it's definitely more uncommon in the media landscape. But I just can't think of any examples of it that are still classified as 'romance'...it's probably more common in "adult" media (like rom-com films, for instance) that I'm not familiar with. ^^;
Like the OP said, young adult media that goes beyond the 'fairytale ending' of romance usually pairs it with an action-packed plot...because perfect romance is inherently dull. :T When you conceptualize a romantic partner as someone who only exists to entertain you and make you happy, rather than an individual who can still have individual goals outside of their relationship, then yeah, focusing solely on that would be boring. There's no story there, just fluff.
Basically, idealized romance as the norm makes people believe that only developing romances can be interesting...hence why stories like (A) tend to stop with the characters getting together, because at that point the fun part is over.
I don't think it's impossible to bring complex romance into the YA demographic, though...despite the fact that it probably still wouldn't be popular. ^^; For example, one thing I appreciate about the superhero genre is how often it makes romantic partners major participants in the story. The intersections of the 'damsel in distress' and 'secret identity' tropes create an environment where a hero making a romantic commitment actually leads to a MORE interesting story, with the expectation that the relationship will be explored, tested, and threatened all throughout. It ain't much, but it's something~