The unasked question, I think is really, what do you want out of this light?
Bottom line, the wife's pocket while out for a walk, and rechargeable, but I've been battling this in my head over and over. My flashaholism is tempered a bit by my tendency to be a cheap SOB. Otherwise I'd be drowning in lights (some cheap and floody MCE/P7 1x18650, some spotty R2's maybe, plus the 2C mag ROP that is waiting for a reflector). I've never counted, but I have at least a dozen. (I know, that's not a "true" flashaholic...however, underlying desire, not the acting out of that desire, is what makes a holic a holic, right?).
My needs seem to be constantly changing (especially after daylight saving time, as evening walks are now in daylight!), and my strong aversion to buying cr123a's has been met with a sudden surge in my pitiful stock of 18650's (after recently making P7 bike helmet lights) since laptop died right after buying a new battery pack for it. With 12 new cells, I want everything rechargeable.....however, you make a good point for backup. This still is a very high probability, as I already realize I can't possibly live on rechargeables alone except for daily-without-problems-life events.
All decisions are made worse in trying to accomplish the most, with the fewest lights (cost) possible. And of course there is no "formula", as there are an infinite number of personal variable that make light selection impossible for someone else to decide for someone else. However,
options are a different story! So the above replies are the exact things I was looking for, and much appreciated.