If your primary consideration is the weight of the battery, the 18650 has the best energy density. If you want something that uses a lighter battery then you are stuck with compromising capacity significantly.
Skilhunt H03.
You can swap the TIR if you want a wider beam:
https://www.aliexpress.com/item/CREE-XML-lens-T5-T6-U2-Lens-Diameter-17mm-Bead-surface-60-degrees-5050-lenses-High/32630479277.html
Stock TIR is 30°, you can put a 60° in it.
I’ve been curious about this lately too…
Some headlamps have the battery pack separate from the lamp, making them feel lighter on your head, but still having a lot of battery power high output and long run-times…
The last one I read about was the Olight HS2
Alternatively you can get long run-times from small headlamps… just use a lower mode…
Looks like the H03 is very competitive in weight
- Zebra with no battery: 1.4oz
- H03 with no battery: 1.55oz
Try to define what you consider long capacity by figuring out what brightness level you require…
Try to define what you consider light weight…
There are other factors in choosing a light, in addition to weight and runtime.
1. weight… this is a function of what battery you prefer and what brightness you use, which determines runtime…
2. cost… ymmv
3. Color Temperature… do you want 4000k or 5000k or 6000k
4. Tint… are you OK with greenish and yellowish tint from cree emitters, or are you going to hold out for the best, which imnsho is an N219b 4500k
5. CRI… can you live with 70CRI, or is 85CRI enough, or are you going to hold out for the best, which imnsho is an N219b @4-5000k w 90+CRI
however, I cannot name an off the shelf headlamp that has an N219b…
since I prioritize High CRI, and Low Weight, and because my application is mostly close range and <50 lumens, I would be very tempted to go with a Manker E02h
this is what I actually use inside my camper as a hands free light, either an AAA Tool or an AAA Maratac clipped to a baseball cap… but obviously Im not running trails with that setup… I have no headband based lamp, so your needs may differ…
good luck with your decisions
bottom line is you should buy more than one, and share your impressions 
A heavy headlamp will ensure a powerful lightbeam, long runtime and a strong neck, all of which are good things. 
Ultraportable, affordable, HID, headlamp, … :-D
Cheers 
On the subject of headlamps, does anyone have any experience with the Convoy head strap? Is it comfortable when loaded with an S2+ or similar tube light?
18650 does have better energy density than anything smaller.
But if you compare 18650 light with 18350 + 2 spares, you arrive at pretty much the same numbers for both weight and capacity. 18350 variant will require extra work to change batteries, but can be used without spares for lower weight.
Nitecore NU20 CRI can be powered/charged via micro USB while using it, so with USB power banks the runtime is limitless.
I’d say it’s probably limited by the size of the power bank 
very good option!
Im a huge N219b fan. Have not seen a beam shot to know the tint or CCT of the LED in the Nu20CRI, but for my needs, which may not be the needs of the OP… I would strongly consider the Nu20CRI.
Now that we are including lights with built in LiPoly power and USB charging… the Nitecore TipCRI becomes a candidate because it comes with a clip that fits a cap…
Or the number of them. 
I have one, but haven’t used it for anything larger than an Hugsby AA and / or Olight S15R (I figure I might as well use my Wowtac A2 or Skilhunt H03 if I’m going 18650 size).
It’s comfortable for AA size lights, but is sometimes hard to get the beam to go where you want it to. There are two elastic bands on each side of the headband to hold a light (or two lights if you put one on each side) plus two bands on the top strap. The side bands only hold the light parallel to the headband, so it only points where your head is pointed. The top bands can be moved back and forth on the top strap, so you can point that light closer to where you want. It’s much easier to use the H03 and just pivot it up and down.
The headband itself is about the same quality as the Wowtac headband, and a little lower quality than what comes with the Skilhunt. I’ve found it much easier to use a light with a side switch such as the S15R as pressing on a tail switch moves the headband around, but then again I’m bald so your slippage may vary. I originally bought it so that I could use it with an 18650 while wearing a baseball cap, but haven’t had a chance to try that out yet.
They should sell them in pairs so you can look like Mickey Mouse… :laughing:
PS - On the lightweight headlight question, I have a Wowtac A2, Skilhunt H02 and H03, and Boruit RJ-02. Of those, the first 3 are fairly light and I’ll gladly deal with a little extra weight using an 18650 to pick up both brightness and run time. The Boruit feels heavy when I wear it, and it’s now the keep in the car for emergency use light.
However, the new Manker angled lights look interesting.
Thanks for this tip, i wasn’t aware of these tir’s. I’m a big fan of the h-03, but the spot is too narrow for me. Now I can remidie that
Do you plan to run with it on?
I have a Petzl Actik and a Nitecore NU20, which are both good for running forests at night (I use them for orienteering).
NU20 has built in battery so is only good for short events (can’t change batrery) or camping, but for those things it’s brilliant for the price. VERY light.
Petzl Actik is much more expensive and twice the weight but you can change batteries. You can buy a rechargeable 1800mah Li battery or 3x AAA. Good unit.
If you want more capacity than nu20 but don’t care about changing batteries I think the NU30 looks like a good compromise. Not an option for endurance events though.
DELETED due to lack of anyone reading, understanding or actually giving a rip