Here is a link that gives a good comparison of the 12V lead (gel cell) battery versus the Nickel Metal Hydride (Nimh) battery, comparing cost, weight, etc.http://www.polmanminerals.com/html/battery_packs.html
The lead gel cell battery is about 3 times as heavy, and cannot be recharged as many times... but I would get the lead battery, still, for the sake of simplicity, recycling, and other reasons.
How you treat the batteries is critical. In a real-world scenario I'd say the gel-cells will usually take a lot more cycles than a NiMH. This is only because the NiMH is much more sensitive to being treated badly. But unlike NiCAD's, none of the mentioned batteries will respond well at all to a deep discharge. They can all be ruined quickly that way.
I use all three types for all sorts of things. But for this application the gel-cell is still the ticket. The gel-cell in my pump is several years old and has several hundred cycles on it.
No matter which battery you choose, it's critical that you get the right type of charger for that type of battery. They have different charge cycles, and leaving any of them on charge on the wrong kind of charger will ruin them. The lithiums are famous for their tendency to explode.
If you want a bullet-proof arrangement, you can get a battery and charger intended for a quality cordless drill. They have their own internal protection circuitry that will prevent them from over-charging, over-discharging, etc.
That being said, I'd personally keep it simple. This arrangement will be cheap and safe:
Power-Sonic battery charger: PSC-12500A $32.34 http://www.mouser.com/ProductDetail/Pow
Batteries: $19.99 http://www3.towerhobbies.com/cgi-bin/wt
Just be sure to stop using the pump when the motors begin to slow down - or charge it even sooner than that.
VERY important bit of info there. I have killed a few batteries by heavy discharge, overcharging or not charging often enough.
You do need to learn how to maximise the battery life and follow that routine with the correct charger.