I'd personally go with a used deck.
I personally think that a more used deck is better all around for cardistry. The cards have been put through the breaking in phase and feel good but are still reliable not to constantly be falling apart in your hands when broken into packets. However, that does not mean that I don't appreciate new deck flourishing, as flourishes will often feel much smoother with a fresh pack of, say, Monarchs or Smoke and Mirrors. Mind you that those are both pretty high quality decks and they are both crafted for and by flourishers though. When handling something like a pack of bikes, I believe that they must be well broken in and used to make your flourish feel good in the hands and look good to the eyes, as the feel of a fresh deck of bikes kind of makes me sick. I don't hate bikes, they just feel terrible when they're first opened in my opinion. So, as I stated in the beginning, I don't feel as if one could ever really go wrong with a nice used deck of cards when it comes to flourishing and cardistry.