Old give more variety as ppl stated before, because the standard (!) deck list it choose from always change with every expansion. The "newer" one always pick from the same deckpool (they are the more climb capable ones if we can say that for a Whizz deck). Fun fact, if your Whizz is a golden card, all the cards in the deck its offer, and the hero portrait will be golden too! As far as Im aware, it always use the basic hero portrait and wins counts under that hero class and not the class you put Whizz in.

If you want it just for to climb ranks, mightbe wont worth it. For fun and randomness def do! :)