Its also super easy with Pokebank to get both versions and trade like crazy between em. Buy one digital one physical, and boom, you have a trade racket going. Starters, legendaries, the works! Sucks about the new update with Miiverse on the 3DS and the region lock, you can't exploit for Vivillions now...
Yeah, the trick to trading is to offer species that are high-in-demand, like starters. So if there's a new region lock update, I wonder if this means you'll always be locked into one specific Vivillon pattern? If so, that means I'll always get River Vivillons when I play X and Y. At least they aren't super-common on stuff like the GTS, where it always seems like Modern, Elegant and Polar Vivillons are the most common.
I love how Vivillon has region-specific forms, it helps encourage more global trading which you need to do if you wanna catch 'em all! (see what I did there)
Completing the Pokédex is such a hassle, in my opinion. Last time I ever tried doing it was in my final run of Pokémon Emerald. I was extremely close to completion, but unfortunately, couldn't get it done in the end. Feebas and Milotic were all the Pokémon left for me to get a hold of, but regardless of how hard I tried, I just couldn't get them. One of my friends offered to give me her own Milotic, but wasn't able to as none of us had link-cable at that time. ):
I remember the closest I came to completing a National Dex was on my Black, which I caught almost all non-Legendary families except Kanto starters, Bagon and Rufflet's line I think. Still, it was quite a hassle having to transfer everything but man it sure paid off in the end. I'll have to see how well my connections go if I want to wi-fi trade in X and Y eventually. Plus, when I choose which version to play more often (aka my main) it usually depends on which version exclusives I like better.