The 24v 70lb will last longer. The 24v system, plus a stronger motor means the system would be working at a smaller percentage of total output. It would be just like you, if you worked at 80% of your total output
you would be done in two hours or you could do 50% all day long. The 55lb would have to work at a higher level of total output. Like Wannabe said, you need a slide ruler or a really good Cow-cu-later.