Most people recommend Flooded Lead Acid as the best balance of initial cost, capacity, and cycle life (charge/discharge). However, I didn't want to get involved in the water maintenance and ventilation issues. I guess the biggest thing was that I didn't want buckets of acid in the trunk, just behind my kids' seats.
Absorbed Glass Mat (or Valve Regulated Lead Acid) are maintenance free, sealed, more or less dry, don't give off hydrogen during charging, and don't have to be mounted upright. They also have good characteristics around current draw, so acceleration is reportedly pretty snappy with AGMs. The downside is that charging a series pack of AGMs requires a more intelligent charger ($) to avoid damaging the batteries, and they tend to wear out more rapidly than FLAs if you draw them down too much too often. These are guaranteed for 400 cycles if typically drawn down to 80%Depth-of-Discharge and 800 cycles if only drawn to 40%DoD.
If you figure that a charge/drive cycle is going to happen 4 times a week, that's about two years if I drive it until it gets slow each time. However, I don't expect to drive it until it creeps each time, so hopefully this set will last for three or four years. By the time they're done, the price for LithiumIon batteries should be more palatable.
For more info on battery type selection considerations, look here . Or here. Or here.
