1) You'll get a slightly better rate at an ATM
2) You'll spend about an hour less in line at an ATM than at a bank.
3) The odds of an ATM hijack are about the same as an ATM hijack in North Carolina, provided we compare apples to apples, which is to say, use an ATM in a bank, bank foyer, the airport or Mega.
4) The odds of losing or otherwise being relieved of your stash of cash before you exchange it are much, much, much greater than an ATM hijack.
5) Ever wonder why there is always one person who has their card compromised at XYZ ATM, but nobody else does, and there have been literally hundreds of locals using that same ATM without issue? I wonder that all the time.
I have no idea how many pesos you're going to need, as I don't know how much you eat, where you eat, and what else you might be inclined to spend money on.
When I use an airport ATM, (which is my very first stop after customs), I'll withdraw 6 or 7,000 pesos to get me started for our 11 day stay in a condo.