Manila-
I have the M3 it's a very nice light and you will be pleased with it. But since you mentioned small as possible and back up, have you considered the Surefire C3? I have both.
Like the M3, the C3 it is a three cell light and has two lamps available for it (100 lumen/60 minutes and 200 lumen/20 minutes).
The deal is that the C3 almost 3/4" shorter and the head being 3/8" smaller in diameter. While that might not seem like much, it is much easier to slip into a pocket and less obtrusive. It would seem better for a backup if you already have a main light.
Of course you pay for this smaller with a different beam, the larger head of the M3 can produce a tighter beam with a brighter central hot spot, whereas the C3 has a slightly more even beam with a less noticable hotspot and a less throw.
The price difference is substantial the C3 being less popular goes for $110-$120 compared to the $170-$210 for the M3.
Both are great lights. If you like I can send you a photo I just took showing the M6, M3, C3, E2e, TW4, ARC LSHF-P, and the Firefly side by side.
GregR