I have no experience with the 51. I understand the 502 is a later model, with "improvements." If I had a 51, I wouldn't necessarily abandon it for the 502. i would say that ZL's entire line of lights is pretty impressive and that extra batteries are rather easy to carry along.

I have been using eneloops, getting about two commuting trips per battery charge. When I take it on a hike, I will probably use lithium batteries.