Simply put, IT DOES NOT MATTER.
You will NOT have a choice. Your weapon WILL be what ever the TO&E says is should be. (Table of Organization & Equipment).
Your assigned duty will determine what weapon you are issued. YOU DON'T GET TO CHOOSE.
There is some flexibility in active combat, and smart commanders allow their troops some leeway, but outside of that, you will be issued a weapon, and that is the ONLY weapon you are authorized to carry.
If you are issued a rifle, carrying a handgun as well (and a private one, at that) is against regulations. As I mentioned, smart CO's will "not notice" in combat, but outside of that, you could get in trouble, even face charges, if the CO is a strict "by the book" type.
"Special operators" might get a choice, regular grunts don't.
Well, err no it does, probably not, no, probably, probably, yes, no, definitely not, they will, indeed, maybe.
Firstly it does matter but; yes you probably won't have a choice. These things tend to change as you move up and down the flag pole. The higher up the pole you are the more options you tend to have. By the time I retired I could pretty much pick and choose out of whatever was available.
MTOE's cover what a unit gets. No matter what the MTOE says the commander is the one who decides what to do with what he has. He can pretty much give anyone whatever he feels they should have. A lot of units have standard way of issuing things but there are also a lot of non-standard things as well. He may do it by duty position or some other metric.
Privately owned weapons policies are set at pretty high level. A commander would have to be well above issuing authority of weapons (normally a Captain) to make such decisions. I don't know many commanders dumb enough to not follow such policies or allow soldiers to do whatever they think is best. Sounds like a good way to get hemmed up.
My last couple of tours I carried a rifle and a pistol. Everyone in my sub unit had a rifle and an issued pistol. In one especially rough tour my rifle I had a M203 under slung and I carried a pistol. The pistol was kind of useless other than it allowed me not to carry my rifle at time that might be inconvenient or inappropriate (say when meeting with a local sheik or doctors at a local hospital). The 203 was useful for smoke, flares and illumination rounds.
I trained in transition fire but never had to use it. I would have to have expended well over 200 rounds and not had any more near by to switch to the pea shooter. The one time I had to employ my pistol the rifle was miles away. Everything you ever heard about not taking a pistol to a rifle fight is true.
Never used a KBAR, they gave me a bayonet, never could figure out what it was for but it stayed in the bottom of my duffle bag and never left it.
So back to the OP's question... If we are just talking combat with some of my closest friends around me then I'd rather have more ammo. I'd get rid of the knife and the pistol and have more magazines. For the realities of going on a long tour in a long war I'd rather have the pistol.
..)