Well, I'm one of those who feels the need to cheat on this one. Why?:
I could see only one long gun, but a handgun can fit in a holster on your person without any extra space (and an additional one, Bersa size or smaller, could fit in your pocket). As far as ammo, well I don't see the difference if you planned to take, say 3000 rounds of 5.56, to cut it in half and use 9mm, .45acp or whatever in the vacated space (thus no more room needed for ammo than with one). What you do get is more versatility, one long gun and one (or two) handgun(s).
In this case I'd take a long gun (probably a 12ga, but see below), and one of my .357mags (not sure between my S&W 65LS and my 6" S&W 586) or my CZ 75 (9mm) or my 1911 in a holster and my Bersa in my pocket.
What long-gun (if cheating) or gun in general (if not) would depend upon a couple factors:
-what is the main concern? Fending off bandits/looters, fending off invading troops, or feeding yourself.
-is the question what is your ideal gun for the situation (assume you have it and ammo for it) or based on what you have on hand?
If bandits/looters I'd want either a handgun (the CZ, 1911 or .357s- it would be hard to choose between them so it would probably be whichever my hand came to first) and a shot gun in an ideal gun question. If only one I guess the shotty makes a little more sense (high powered) if you will be hiding out in the middle of nowhere, if you will be spending time in populated areas a handgun would be better. If this scenario is based on what is on hand I'd have to go with my 9mm (CZ) as I have far more ammo for it than any of the others (my SKS has a decent ammo stash, but it wouldn't be my choice for this situation) and I don't currently have a shotgun.
If fending off invaders I'd want the 9mm CZ (as more armies use 9mm than anything else so there would be more chance of resupply) if I could also have a rifle. If we are again going to be strickly holding to one (or for the rifle to go w/ the 9mm) I'd want a bolt rifle in a decent caliber (.308, .270, 30-06 or similar) and a good scope. Against trained troops few, if any, of us would stand a chance alone (or with one or two buddies) in a firefight (remember they have numbers, training, communication, and the ability to call in airstrikes and artillery). My goal is to live. I'd want to be able to engage at longer distances and get away. As I don't currently have one and if we are limited to what we currently have I'd go with a .22 (my Marlin 60, Ruger 10/22 or my bolt action Rem 581 would all be ok) and engage at shorter ranges and count on the limited report and flash of a .22lr making it tough for them to locate me.
If feeding is the main concern I'd be less worried as I would use my fishing gear for that and the gun would be mainly for SD situations (as they are now). If only one SD gun in a non-carry situation I'd prefer a shotgun, in a carry situation I like my revolvers, CZ and 1911 but I'd probably go w/ the 65LS (or for space and ammo space reasons I may go with the Bersa). For a hunter (which I'm not) a shotgun or .357mag (esp. the 6") make sense as either can have its ammo adjusted for anything from bear down to squirel (the shotgun may be even better, if you have one, since one can also go for birds).
