Battery question - Lead acid vs ni-cad, or other

[Charles Freeborn]

I've got an electric bicycle that uses a 24v (2-12's in series) battery pack. Currently and originally it holds a couple of motorcycle type batteries that cost about $40 each. Needs new ones now. So, before I go that route (again) would there be any advantage / cost savings / performance improvement in converting to ni-cad or other type? I'm okay buying individual cells and constructing the pack myself.
Thx.