2xR123 or 1x17670 for Malkoff M61?

Hey guys - forgive me if this is an ignorant question, but it's been a little while for me with electronics. I currently run an M60 on 2xR123 cells. Soon, I'd like to pick up an M61, and I'm wondering whether I should run it with the same bat configuration, or with a single LIon cell. Not sure which will give the longer run time (a 17670 has a much higer capacity, but lower voltage); my only complaint with the 2xR123 cells is the lack of warning...the light never dims. When the batteries hit their voltage limit, they shut off -- before the M60 drops out of regulation. I try to recharge them before this happens, but sometimes don't get to. Probably not good for the batteries, and no warning for me. What do you think? Thanks in advance!