4S LiPo (14.8V) is the way to go. You can get tons of cheap LiPo pack all over asia stores like unitedhobbies or through ebay.
LiPo is light and gives way more punch than NiMh.

I use Zippy-H 5000mah pack in my truggy box and True-RC 8000mah pack (actually made it from left over cells) in my buggy box. Both are little bit overkill as any decent 2000+mah will do, but at least I can charge them once a month.

I am done with NiMh as they just have so high self discharge rate.

The only NiMh I am ok with are in Glow Ignitor as my 3S LiPo powered powerpannel sometimes does crazy things.