The reason I went with a Niterider Solas (Li-ion, USB-rechargeable) for the rear and later (last month) for an Ixon Core 180 for the front, is not because something might have larger "capacity" (you can have either type of battery in whatever size you want in theory), but because NIMH delivers a mere 1.2V per cell, whereas as Li-ion delivers a 3.6V per cell. This means it can push more power to an LED for the same weight than a NIMH can, which results in a much brighter light than would be possible with NIMH cells. As a result, my SOLAS rear light is really, really bright, and the IXON Core is bright enough, and much brighter than a comparable-weight NIMH front light (or, for that matter, most dynamo lights I have seen - especially the rear light).