This is from personal experience of using the UKE mini Q40, so there will be variation and YMMV. I have not had the chance to test fire the Q40 and Arc-LS side-by-side, though I have been priveledged to play with one thanks to Hotfoot.
Bulb life of the Q40? I dunno. Pretty darn long, though definitely no where near as long as LEDs are supposed to last. I had one for about 4 years, through my army stint and in to my university years. I hadn't yet changed the bulb when it finally got stolen/lost/misplaced. (Funny story that I might relate somewhere else.) Used it to help recruits find lost stuff, for fire control safety and a couple of others chores. Used in in a few all night crab hunting and fishing expeditions, so there are quite a few burn hours behind it.
Which brings us to the next question. How long do the batteries last? I usually average 3 battery changes per night of fishing. There is still quite a lot of light left, but you need to be able to shine out to see where the line is going and if the fish is going to get you caught up in rocks or other angler's lines. So a change after every 2 hours or so of use and the light is probably as bright as the Arc-LS even at that point.
Which shows up the next point, that it starts out brighter than the Arc-LS, especially if you're using lithiums. It does give an incredibly brilliant white beam with a hint of yellow, something like sunshine. But by the time it reaches 3hours of use on alkalines, the beam is considerably more yellow.
Could you run over the Q40 with a wheelchair? Try a car, or an army truck. I believe the ads used to show it being run over by a car. The one I lost was actually run over by an unladen army truck of the 3-ton capacity type. (That's a long story too.) Of course the truck ran over the whole thing, lexan lens included. No noticeable cracks, though quite a rough surface was imprinted on to the body of the light because of the asphalt. At a much later date (when I entered university), I used it as a dive light and noticed no leakage.
Which of course brings up yet another point. The Q40 is fully dive capable. I don't recall Peter giving that reassurance for the Arc-LS. How water-resistant is the Arc-LS anyway?
Finally some other quick points as this post is getting too long:
Cost. I can get a Q40 for about SG$25. IIRC, the Arc-LS goes for US$160 for the full kit. That's quite a few Q40s.
The Q40 is simple and robust. The prime component (lamp assembly) is quickly replaceable at low cost.
Batteries. Sure, the Arc-LS can take quite a few battery configurations, but that is accomplished through changing the battery packs. If you really wanted to argue the point, all that the Q40 head lamp needs is 6V in a battery format that can deliver high amperes. (Hmmm.... got an idea for a super bright headlamp using a large battery pack, PWM regulation and 4 Q40 lamp assemblies.
) So you could conceivably build battery packs to take 2 CR123s, or the 2CR5 or whatever other configuration you want.
But when all is said and done, these 2 flashlights are great and robust and probably among the best in their class. The Arc-LS comes with a "Wow" factor for the technical minded, but a huge price tag. The Q40 has simplicity and price running for it. They both blow the mind in terms of brightness.
And personally, even having a Q40, I still want the LS because I can envision running it on single CR123 so that it will be small enough to fit in my jeans, though I will never be able to take it diving or fishing as the thought of losing something so expensive is difficult to stomach.
