Nimh packs are smaller, so you can fit them into spots on your bike easier, which also makes them a little easier to transport off the bike. You would need a smart charger designed for your Nimh pack as well, which are more expensive, but you want to get smart so you dont overcharge your batteries.
They are way more expensive, but there is a flexibility to them. If you want to overvolt, you just make your battery packs a little bigger(by bigger I mean overvolting for increased light), 11 or 12 cells instead of ten. You can also take more of them with you and make more as you can afford them. say you start with 5ah which is 2 packs of either 10/11/12 AA batteries. Down the road you could make a couple more and wire them in. With a Sealed lead acid, your adding another monster to increase it or dumping the old battery in favor of the new. Often, the Nimhs can be recharged more often, but im not sure of that completely. The big thing about sealed lead acid is its cheap, and doesnt have the trouble with overcharging like the Nimhs. Im going with lead acid, until I have extra cash to make the Nimh packs, but when you can afford it, that is the way to go.
If you understand them and now how to run a system with them, the lithium system is even lighter. Sadly, the lithium-ion can explode, but thats just a small thing.
