I have money for 40 pack of one expansion. I have no idea which is better. I have only few cards of each, not many legys, only thalnos, leeroy, mooneater and that's it. I mainly play shaman random cards I have, odd rogue because it's cheap and odd pala same reason as rogue. My fav is shaman and rogue. What do you think, which expansion will be better to load?
There is also a consideration of how long you have been playing, how much money you will have in the near future for packs, and the real state of your collection. For the last part of that you should install Innkeeper and/or make your collection public on Hearthpwn. That will let others see your collection and make good suggestions based on what you have when asking collection/deck/crafting questions.
If you are in your first year of playing please read the post linked in my signature.