>>14413171The story differences in X & Y are only related to the Team Flare I believe.
>>14413163Based Josh. He initially said that Serena was the one who gave him her Pokemon, as
>>14412968 points out, but doubled back on it, saying that he meant "the little brown one".
Then the Italian Reviewer stated that whoever starts off with the Pokemon WEAK to your one, will give it to you later. We now know that this is Shauna for both Male and Female players, and there hasn't been any noticeable dialogue changes between them in either of the leakers playthroughs.
So yes, it seems Shauna is waifu of the year, regardless of who you are playing as.
Also, Serena/Calem act as the main Rivals of the game, and are the only characters in your circle of friends who actually battle you multiple times. No lovey-dovey stuff there I'm afraid, it's a purely competitive friendship.