Should I favor a tighter joist interval or more blocking between joists in order to get the most firmness in the deck?
Give a short and simple answer because you have a relatively common deck size.   16&quot; OC is fine.   Put two rows of blocking with the same materials as the joist.   The blocking should split your deck in thirds.   If done well the blocking will appear to be some sort of magic beam that goes through your joists.
I mention that because I always add at least one row of blocking to all decks.   You do not want your joists turning on you due to moisture/humidity issues.   The best way to combat this is blocking.   Therefore you need to do it anyway and with 16&quot; OC you should have little to no deflection give you are using at least 2x8 on top.
Note:  I want to add that your piers probably play a bigger part in the apparent &quot;stiffness&quot; of a deck.   Any kind of sway in them and your deck will have a bouncy or flimsy feeling.   This may mean that you want to sink your piers a little lower, get bigger piers/lumber and ensure that they are set correctly.   If these things have any given your deck will always have give.