Whenever I start a new generation, I only trade within that generation. I've traded between Black and White for all the version exclusives. Now, with White 2, I'm in the (very slow) process of trading over the 151 Unova Pokemon from White. Since White 2 is a continuation for me, I'm trying to have my Pokedex accurately reflect the cummulative experience through both games, and focus on catching the 150 Pokemon that were added to the Unova Dex.