I think it's quite easy to me. I'd buy DF, but since I already got many DF cards (since I think it's the best set) I'd rather buy DP, since I haven't got a single card from DP yet and the Pokes and trainers in it are quite useful.
save your money for dp3 and then you might be able to get 2 boxes.
I got a dp box recently and the cards are okay but i think that i should've gotten df.
Go for past sets because after 3 months-6 months the sets are harder to find and you can always get D/P set later. Try to go for EX Ruby/Sapphire if you can and if you can get Wizards cards definetly go for those since they don't make those sets any more but they do still print off each set that was released already.
They are hard to find except online and I have over 1,300 of the Wizards cards and 1,600 or so of the EX series set cards and a toltal of about 2,951 Pokemon cards. Note you can still play with the old rules at home but just make sure you use card sleeves so you don't damage your cards.
you know really in the next format PK is going to step up and make some sweet combos with the new sets... so i would snag the PK early... so i say PK or DP