If you do a great deal of night riding, such as many nightly commutes per week, you should get a NiCd battery-based system. If you do only occasional night rides, you'll likely do fine with a SLA-based system. But in either case, you will probably be very wise to invest in a third-party battery charger, and not use the battery charger supplied with the system you buy.

NiCds rechargeable battery, if properly cared for can be recharged 3 to 5 times as many times before they wear out as can be SLA batteries. Cost of a high quality third party charger for either system is roughly the same. Overall, NiCd batteries are at least as inexpensive and probably actually somewhat less expensive a source of power than SLA batteries if you are using them frequently, over the course of their total life.

If you are using the battery infrequently, for, say, 20 rides per year, then the more expensive NiCd will probably die due to its shelf life expiring before you use all its available charges. NiCd batteries are, overall, about 30% lighter for a given amount of power capacity than SLA 18650 batteries. This is a significant, but not utterly overwhelming difference.

SLA batteries retain nearly their full charge for two months or more just sitting on the shelf, unattached to a charger. NiCd batteries lose about 1% of their charge per day when sitting on the shelf, due to internal self-discharge.

NiCd batteries have a flatter voltage vs time curve during discharge than do SLA batteries. This means your lights will remain relatively more constantly bright during the entire useful discharge life of the battery with a given lighting system than would be the case for a SLA battery of comparable amp hour capacity and voltage.

Both NiCd and SLA batteries can be severely damaged by being deeply discharged to down below 75% of their rated voltage. With either system one must never run the Ni-Mh battery into the ground, letting ones lights go from yellow to orange to dim orange. Turn your lights off when they get noticeably yellow. Otherwise, you risk permanently damaging your battery.