There is no "in general", though. If you're keeping it specifically to stash away for duty use, I would very much see what my gun likes, because accuracy is all that really matters in this context, since the terminal ballistics of both are pretty mediocre. The only real difference I can think of is that M855 can penetrate UHMWPE, while M193 out of a 16" is not going to be defeating steel plates very well. However, accuracy out of my gun would easily trump any body armor defeat, given that there's lots of ceramic armor out there.
All things being equal, I would lean toward M855 for pure duty use, I guess, because of its slightly better ballistic co-efficient and UHMWPE defeat, but I would much rather buy M193 if I was simply stockpiling, so that I could use it for training and use it against steel targets with less concern (assuming proper distance to give the bullet enough distance to be safe with a 55 gr bullet). If this is truly an emergency use only stash, I'd much rather have a smaller stash of quality duty ammo with barrier blind bullets than a bunch of M193 and M855; going through 1000 rounds without any kind of resupply or movement is pretty fantastical.
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