Nimh or Lipo for King hauler?

[Cooper]

And,, if you are not familiar with lipo, li-fe, or such, you will want to purchase a decent or very good charger. 99% of lipo problems are from overcharging or not being careful with pack. If damage they are a time bomb waiting to swell, burst. Just do some research and take care of them and lipos will outperform (apples to apples) nicad, nimah, almost any other chemistry for our uses.
Lipo, lithium polymer , cells (1s, 2s, 3s, ect) are measured (voltage) per cell then pack voltage. The main concerns (brief) with these are not to let voltage (resting) to drop below 3 volts per cell. Very good rule is 3.2 volts not 3. Never , never try to overcharge. Cells need to be balanced in a pack usually the last 3~5% of cycle charge. Don't let get damaged, dropping, smashing, altering, saltwater, water( ok but stop using and place in bag of dry rice overnight) look around as most batteries are rebranded as only a handful of actual manufactures exist. And sometimes the lipo cut off circuit will just stop movement or slow down and then resume, if you notice that, stop! Charge.
And I run lipos too, don't know where or when my last nicad was used. As far as what sort, in our trucks the c-rating, (ability of battery to output power) doesn't need to be that high. And not one battery company on the market can truly live up to the c ratings as advertised. (Referring to the high end packs. )
Some good lipo reads. http://www.rcgroups.com/forums/showthread.php?t=1577989

[Supermario]

I started with Nmih in all my trucks. Then I got a loader and due to size and space constraints it came with a lipo. I'm sold on Lipo ever since. Everyone has given sound advice and cautions with these. The main reason I use is not just for the power. It's the reduced size that makes the difference. With all the electronics we add and have in our builds, the less space taken by the battery the better! I couldn't have my day cab the way it is without lipo.
Here's a size comparison for you. 2,000ma Nmih compared to a 2,200ma lipo.