I'm leaning toward Y because a) I like the bird and b) back during the Red vs Blue days I was hardcore Red and, well, Y is red lol ;-)
That being said, it's really too soon for me to say for sure. I was nearly positive that I was going to get Black, but then I heard about the White Forest and ended up getting White/White 2. I wouldn't be surprised if some similar game feature swayed me again.
I am leaning towards X right now because I like Xerneas. I practically yelped when I saw both Xerneas and Yveltal as they are both based on Norse mythology, which I really like. That and Xerneas may be the first ever legendary I actually like. I will have to buy a 3DS first though. It's funny because I will probably only ever buy pokémon games for this new 3DS, never anything else. My final decision on which version I get will be determined by the version exclusives. The real decision for me is which language to buy the game in.
I'm having a whole lot of trouble deciding which version I'm going to get. On the one hand I am madly in love with Xerneas's design. It's beautiful and amazing and freaking majestic. On the other hand Yveltal is a freaking beast who demands all to kneel down at his greatness or be snatched up by his scary tail hand.
I'll probably end up going with Xerneas and whichever version he comes with (more likely than not, X. But you never know!). But we'll see what the version exclusive information has to tell me beforehand.
When the games are released, there will be anywhere between 4-5 copies of either game in my house, Between my two sisters my brother and myself. As of now I plan on getting both for myself, but if I had to chose one I'm leaning towards Y, but it would ultimately depend on exclusives.
I know I'll get both games but as for which one will be my "main" game that I keep a save file on, I'm thinking X and then Y will be used for Pokemon training and gaining more special items that might otherwise be a pain to obtain (i.e. Master Balls). It also means I'll be able to trade many more Pokemon and not really need much in return.