If I'm honest...its hard to decide, because normally I would go for Alpha Sapphire as Water is my favorite type next to Dragon, Fire, Ground, Rock, Electric and Grass. On the other side, having seen Groudon's new Ability that makes him immune to Water-type attacks isn't bad either, so Omega Ruby might be a good choice too, not to say, it contrasts well with Pokémon X which has a blue theme from the start. - Blue X, Red Ruby
Regardless of version though, I'm picking Treecko, as I already have used Blaziken a lot, and I'm not too interested in Swampert. In the end, however, for me, the version I will pick, will depend on the version exclusives (barring Legends) and their availiabilty in shops, as I never buy both versions. I see no reason to, because I don't have two 3DSes, so its redundant for me to get two of the same games with minimal differences. This way of thinking has always been around for every generation of Pokémon Games
Besides, I enjoy trading Pokémon with others. Its the very essence of the games after all.