Of course, we all know that when two versions of a Pokemon game are announced, they are similar, but they each have differences to each other.
And such is the case for X and Y; both games will have differences in their Pokemon distribution, city design, items, trainers, gym leaders, and some other stuff I can't think of now.
Additionally, we should all take note of the fact that, over the years, the differences between each version are becoming greater and greater, so much so that (if iirc) it was one of the reasons we had two sequels instead of one remake for B/W. So, how different do you want X and Y to be? I myself prefer something like D/P or R/S, just because it gives a better opportunity of having only one sequel/remake (which I assume to be Pokemon Z.)
The most common version exclusives are the box art Pokemon themselves. However, this time, the two mascots do not seem to be found in the same location (because of there different designs,) which may mean that the Version Mascots are not exclusive as we would have expected (like G/S and HG/SS.)
So, what do you think the differences between Pokemon X and Pokemon Y will be? And to what extent do you want them to be different?