Same thing with MD3 2 x 18500(1500mah) vs MD4 2 x 18650(3400mah).
The 2 x 18650 give at least twice the runtime there too, even at the same 8.4v.
However, I find that the MD3 is a more convenient form factor, so I just carry an extra set of 18500 in my pocket. I have my HD XML2 on an MD4, but...