Ultimately, it doesn't matter which card has more acceptance locations--what matters more is what is accepted when you go to make a purchase at whatever merchant. I always have a Mastercard, Visa, and an American Express card in my wallet. Just the other day I was in the local dollar store, and they only accepted Visa, Discover, and ATM cards.
I have seen merchants accepting only Visa, those accepting only Mastercard, and those accepting only American Express. I haven't seen any accepting only Discover, but there's probably a few somewhere (thought I don't have one).