If you plan to mostly use the medium and lower brightness selections, then the higher mah AW (Panasonic) 18650 batteries like the 2900, 3100, and 3400 will be best.
However, if you plan to mostly use the high and turbo settings which draw high current, then you will get best results from the AW 2600 mah 18650.
I realize that this sounds unusual, but the chemistries are different. The 2600 is the older LiCo chemistry, and that holds up to higher drain better, and holds up its voltage better in those conditions. It will hold the high setting brightness for longer than the 3100(and probably the 3400 too) before it starts dropping off.
The newer chemistry with the Nickel in it can last longer at low current draw, but drops voltage faster at high current, so the run times on high or turbo won't be as good with the new chemistry that is used in the 2900, 3100, and 3400.
So, for a LED like the XML that likes a lot of current to run at high power, the 2600 is still the battery of choice.
If you want to mostly use the low modes, then get the 3400.
If you look at the discharge graphs of these different batteries closely at the high discharge rates vs the low discharge rates, you can see what I'm saying.