I have kind of a weird question.
Which will be more stable headstanding, a P60 light, or the 18350 version S2+?
I currently use a P60 light as my bedside light, but I would like to replace it with something smaller. However, I won’t bother if it’s going to fall over everyime I bump the nightstand. Does the shorter length of the S2+ compensate for the smaller base?
I don’t know the answer to your question but this is a sort of what we in Sweden call “dagens i-landsproblem”. 
Try a C8!
I don't have the S2+ but I have several lights, including a 504b P60, several S2/S3/EDC-type lights, and a compact single 16340 EDC-type light (602C) and they all exhibit the same problem (House-of-Dominos-Falling-Effect for you mixed-metaphor types) when I accidentally bump into them.
An alternative might be the 501a style of 1x 16340/18350 P60 host. In that case, the squat shape with the relatively larger reflector seems to actually help some.
Here's a compact P60:
...and an alternative: The AB-T60, which I've always wanted, but is just too expensive for a novelty light. It's another compact single-16340 sized light, but with a large reflector/head that would be very stable in a head-down stand.
The S2+ 18350 is super stable on its head, partly because of where the center of gravity is. (Pilots are big on CG, right?) It tailstands ok but is a little bit unstable owing to only two 1/2 inch long tabs for it to rest on.
Edit: I use a SWM C25C on my nightstand, mostly because of the nice blue trit in the tail, but I manage to knock it over every day or two, waking everybody up….
+1 - I got one mini S2+ with a triple and one with a UV LED in it - both are very stable on their heads. I still got one more mini S2+ host, figuring I'd go for a neutral or warm TIR optic maybe. These are nice little lights for sure... The stability is a great feature of these lights, much better than P60's or full 18650 tube lights.
Sweet. Looks like I’ll be ordering one!
I check the stability of the flashlights (and also a Solarforce L2M short) head standing on a flat plane. I tilted the plane until the flashlights tipped over and here’s the results:
Solarforce L2P: 9.1°
S2 mini: 13.3°
Solarforce L2M short: 15.8°
All lights were tested with a battery installed.
Wow that’s really significant. Thanks for doing that, great test.