so does it make more sense to get this R5 iteration or the new(er) XML version?
The XM-L will be brighter on 1x14500 sources, but max output will likely be little changed on standard batts. Overall efficiency is also similar between the two emitters, except at the higher output levels on XM-L.
Throw will definitely be lower on the XM-L, compare the XP-G, for equivalent output levels.
I have a V10A(XM-L version) and found that the light with an Eneloop flicker continuously when the battery voltage drop to a certain low level(don't know the voltage).
Also the light with a 14500 li-ion just go out without any warning(indication) like an Eneloop when the cell voltage is about 3.0V.
Looking at my runtimes, it does seem like the my XP-G version may have flickered as the battery was nearly exhausted. But this is unlikely to be a low voltage warning - that would have been visible as a series of flashes much earlier in the run (which I do not recall seeing). It is not uncommon for a lot of lights to flicker as the battery is nearly dead - this seems to be a circuit issue as it tries to draw power from a nearly exhausted source.
As for cutting-out on protected 1x14500, that is perfectly normal - most well-regulated lights tend to do that on protected sources. It is only on direct-drive lights where you may have a long drop-off period before reaching the protection cut-off.