The metagame is a LOT faster. Claydols provides consistent drawpower, but if you can afford them, Uxies are a lot more useful.
The meta right now will change drastically after Rising Rivals comes out. As of now, it's mostly G Decks, whether focused on Palkia G, Dialga G, or both, Gengar, Machamp, Rampardos, Giratina, a few GG variants, and other rogues.
Once rising rivals comes out, expect a lot of Hippowdon, Flygon, and Arcanine. Also, Porygon Z and Garchomp will/are grow(ing) in popularity.
If you have to buy, look into Flygon and G stuff. Although most of the G stuff isn't hard to trade for short of the Xs, and even those aren't horribly difficult, plus they've gone down from their original pricing points.
Uxies/Claydols can be a pain to pick up, but are definitely needed. I'd suggest the Uxies as Claydols will probably rotate out soon.
Also, play Unknown G, if only to prevent Macheap from donking you with takeout.
If you any other more specific questions, I can try and help by PM. I myself am adjusting as well and this is what I've noticed, at least.