RE: NIMH vs. NICD
Each type of pack has it's positives and negatives...
NiMH:
Usually have higher mah ratings, giving longer runtimes, but they take longer to charge. A "matched" NiMH will give a phenominal runtime. They are more susceptible to extreme temps and they weigh more than NiCd.
NiCd:
Cheap, light, durable and fast-charging. Short runtimes (lower mah ratings), most are not "matched".
You got the right idea, simultaneously charging and running. If you're having fun, that's all that matters...