Is there a reason to get the 3.7-6volt WE Cree drop in vs. the 3.7-13v?

Is there a reason that you would want less versatility from the bulb?

If you want to use one RCR123, or a 18650 cell, the 3.7 - 6 Volt WE is a must.
The 3.7 - 13 Volt WE does not stay regulated below 3.7 Volt.
And the 3.7 - 6 Volt WE 170 Lumens drop-in is very effective, gets great output and runtime on a single RCR123 cell!

Actually I should ask, if I do run the light in 2xCR123 configuration with the 13v it won't be worse than the 6v with 2x123, right?

It should not, as long as your voltage is well within the specified range where the output is regulated (and if the voltage is falling too low with these combinations, your batteries are completely empty!)
I use both the one 18650 and the one RCR123 cell with my WE 170 Lumens 3.7 - 6 Volts drop-in, and I am very satisfied with it.
I also use the two-stage Mc2E switch, 22 Ohms, for a low and a high output mode.
These are my two different lights with the WE:




(the one RCR123 cell light with Lumens Factory 3.7-6 Volts 170 Lumens drop-in to the left on the last photo!)

Cool, thanks.