Like anything else it depends on the mission, in this case what "SHTF" means.
What's the scenario?
1. A post-atomic holocaust? In that case, any ammo I can get is good, even (gasp) Olympic.
2. A post-hurricane type situation where law and order breaks down for anywhere from a few hours to a few days?
3. a scenario where you will need to use deadly force to protect yourself/family?
4. a combination of the above?
What's the threat and the target?
My answer for #2: random marauders, possibly armed, possibly not
My answer for #3: some probably armed criminal type
What do I need to do to the target?
2. get 1-5 dudes to GTF away from me. At least a few shots - IF I'M IN REAL FEAR OF MY LIFE - are necessary. This is the one situation where I think a civilian may possibly need to expend several mags - depending on the situation.
3. essentially put down the dude ASAP.
How far away are they going to be from me?
Hain't that an interesting question.......... Let's put it this way, how likely is it that they'll be more than 100yd from me before I'm justified shooting at them?
What kind of ammo would I like to have?
2. at least several 100 rds. of something. For the scenarios listed above, I don't see any difference between XM193 and XM855
3. 30rds. worth of BH 75gr. Every shot has to count.
Bottom line: for #2, either XM193 and XM855 will probably be just as good. Keep several hundred rounds at least. If you're worried about #3, keeping a 30 rd. mag with BH 75gr. seems like a good idea.
[ETA: a few phrases and typos here and there]