Since guns without ammo are just expensive paper weights, I would have to argue that ammo is much more important than guns (if you already have a few guns). Properly maintained guns last a pretty long time, and I'm willing to bet that most people's guns will outlast their ammo supply. Besides, you can only shoot 2 guns at a time.
If the poop ever does really hit the fan and you need to evacuate, you will be highly limited in how much of each you could reasonably carry. For me, I would grab my AR-15, my shotgun, my .22 Buckmark, my 1911 as my primary pistol, and my Glock 26 as my back up gun. For me, a reasonable ammo supply would probably be 300 rounds of .223, 100 rounds of buck shot, 500 rounds of .22 LR, and 250 rounds of .45 acp and 9mm. Durability and reliability would be paramount in deciding which guns I would carry. Carrying more than that would weigh you down a lot and probably take the room of other essentials.