Well, if you're looking at a 6 cell(7.2 volt) NiMh or a 2-cell(7.4V) LiPo, honestly there won't be enough of a difference to notice. The advantage of LiPos is not more speed, it's high capacity in a package that is considerably lighter(LiPo's typically weigh about 60-65% of a NiMh of the same capacity), & low electrical resistance such that you only need one LiPo pack & you can recharge it immediately after a run(whereas NiMh's need a little cooling down time first, about a couple of hours, so to do a lot of runs in a day, you'd need several NiMh packs). But speed will not be a significant difference in general....