Well i take it we're talking about "AA" batteries here. Alkaline disposables tend to have more "thereoretical" capacity (ie high Ah rating) than NimH. However, NimH can sustain current drain better than Alkaline and may in actuality last longer. It all depends on your equipment. Oh and temperature, NimH is much better in colder temperatures.

If you're burning through allot of batteries i would go rechargable, you'll see the cost savings fairly quickly.