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one thing to know is that if you intend to fly with the light the LM is in a grey area of legality with flying. Their packs are individually certified outside of the canister but they are not certified together as a unit so those larger packs are definitely in an area where you may get away with it when others have not gotten away with it.
UWLD's biggest battery is fully legal to fly and intentionally kept under the 160wh limit as well as being engraved on the bottom of the canister with the wh rating and UN38.3 compliant to try to minimize any hassle with airport security *it is often countries outside of the US that have posed problems for people*.
At some point though you do have to ask at what point is it too much or unnecessary. The big battery with the LD-40 is what I have, actually have 2 of them...., but I am regularly doing 4-8hr cave dives. I also have the ability to add my heated vest controller to the stack cap when I want/need it and fully remove it when I don't which is quite useful when traveling or if you regularly dive in different thermal situations where you would/wouldn't necessarily want the vest controller. With the others they are permanently attached to the head so you always have to contend with the EO cable. It's not that bad to deal with but you do have to deal with it.
This link is a comparison of 2 generations ago lights from UWLD and the LD15 beam will give you a reasonable view of the LD40 on low and the LD26 a reasonable view of the LD40 on medium or the LD20 on high. The LD26 had a wider beam angle than the LD15 and the current LD20/40 are more comparable beam pattern wise to the LD15 though considerably brighter. Doing these comparisons is incredibly difficult on the dive team to ensure variables are minimized and unfortunately surface tests don't give a good enough depiction of what a light will do in the water. The UWLD optics have been custom designed for use underwater and that gives a very large difference in the beam between the surface and the water. LM/Halcyon optics are not in the same league which is part of why their light heads have to be so big to get the beams focused in properly.
Stepping back as an engineer and looking at the lights objectively and trying to put my personal bias to the side.
Trying to compare these lights is properly annoying because they don't use the same units of measurement to compare. Lux is highly variable and is more comparable to horsepower to measure a cars power output than measuring in lumen which is more analogous to torque *hint, motors are always designed in watts of power which is torque and HP is a function of torque so we are going to use lumens*. Batteries have historically been called out in amp-hours of capacity but that is a function of nominal voltage so we are going to be using wh to compare batteries.
@Bobby quoted the rough burn times with the big battery. The UWLD battery packs are built as either 1x, 2x, or 3x sets of packs so the mini canister is the burn times/3, and the medium is the burn times *2/3.
Halcyon refuses to disclose their lumen numbers *largely because they are very low* and instead is adamant that lux is more important because they have their beams focused down to a laser so they have a very high lux:lumen ratio. What we know is that their 5.2a battery is ~55wh of which 50wh is going to be usable and on high it burns for 2.5hours. 50/2.5=20w consumption and the most efficient LED's out there are ~120lumen/watt which says optimistically it's 2400lumen output *realistically it's actually about 1500lumen, but to give a direct comparison to UWLD it's the LD-20/mini.
Light Monkey for some ridiculous reason not only still quotes their battery packs in Ah, but also quotes their lights in watts which is ridiculous but whatever. The LM 20 is roughly the same power consumption as the LD40 and is quoted at 4770 lumen but the inefficiency of the optics makes the LM32w seem more analogous to the LD40 in the water but we'll be using the 20 for comparison since the quoted specs are comparable.
What is important is that the optics on both the Halcyon and LM are less efficient than the UWLD which is why their light heads are so big and it means that even though the light output at the emitter may be similar, the light going into the water is a lot less. This is pretty obvious when you have them in the water together.
So if we are to compare like lights.
Halcyon Focus 2.0/5.2ah-$1660 as of time of post from Extreme Exposure. ~1500 lumen, ~2.5hr burn time. Big head, but has variable focus. No support for heated vest. IMO best hard goodman handle on market. Magnetic toggle switch on the light head.
Light Monkey 10-20-$1430 at time of post from Light Monkeys web store. 3.5hrs burn time but 2x battery size vs. Halcyon *smallest battery available*. I really can't think of anything that would be a tipping point to buy this light, especially given the really bad history of the 32w variable with reliability and flooding since it was released.
UWLD
LD20-Mini $1547 at time of post from UWLD Website. ~2.5-3hr burn time. $100 cheaper than the Focus 2.0, has support for a heated vest or video head *though with the baby battery it won't burn for very long*. SECS charging so the only time you ever open the light is to add/remove stack caps or to fly. Tiny light head.
LD40-Short to compare to LM10-20. $2145 at time of post from UWLD website. ~2.5-3hr burn time. Quite a bit more expensive than the LM, and you lose variable focus. You do get a light head about a third of the size, you get the latchless design and SECS charging, and the UWLD has proven to be one of the most reliable lights on the market. Please note that when compared to the 10-20 from LM, they use essentially an identical battery and the LM burns for 30mins longer. Unless it has a more efficient emitter *which it does not*, then the only explanation is that it is consuming less power and is therefor producing less light despite claiming to put out 20% more light than the LD40*. The 4770 lumen may be a theoretical max on the emitter at the power they are producing but does not account for any output losses where the LD40 is a conservative amount of lumens through the glass instead of at the emitter*.
It is very difficult without a background in engineering to really figure out the muck that is the light market because the light manufacturers try to use units to position themselves ahead of the competition or at least appear to, while also clinging to specs that are no longer relevant with todays technology. Unfortunately quoting things like Lux but refusing to disclose lumen can tell you how intense the light is within a small circle 1m from the light head but tells you nothing about the total amount of light being put out. Quoting the theoretical max lumen output at an instantaneous point which is something that companies like Big Blue do and having their light output fall off almost instantly vs. quoting the average output *the 3 companies above use constant output emitters so that's not an issue with these 3 but is certainly something to consider with all of the Asian import lights*. Quoting the theoretical lumen output at the emitter itself vs. the lumens out the glass which have all of the efficiency losses factored in, etc etc. It's a very complicated world and it's somewhat similar to the car industry back in the day when companies started quoting brake HP instead of wheel HP, and then now with electric cars trying to convert that to torque at the wheels which is what you actually experience as a driver, it's all immensely frustrating and without an engineering background is almost impossible to filter out.
I hope the novel above not only helps @acreichman make a decision but also can help enlighten others to make sure they are comparing apples to apples for lights because it is incredibly easy to believe some of the marketing nonsense and make an unfair comparison.
