Thanks guys.
What does it mean to be regulated on that battery?
How will that affect use?
An A8 will give you steady output until the 18650 battery is almost completely depleted. Then the output will drop noticeably.
On an A9, you will get slowly diminishing output the longer you use the light with an 18650. But on the plus side, you get longer runtime on the same cell than you would with an A8.
The lights are related, but are two different creatures. Although both can use 18650 or CR123 cells, they react differently. An A8 will give you regulated output with an 18650. But the runtime with that cell will be a bit less than with an A9. Also, you have to deplete two CR123 cells for a few minutes on low, in the A8, before they will work on High mode.
The A9 doesn't require a few minutes on low before the CR123 cells will work on high. You get a bit more runtime with an 18650. But it won't be regulated output.
Generally, if you're more likely to use an 18650, get an A8.
If you prefer CR123 cells, get an A9.