Battery confusion

[Angelus Lupus]

OK, so after reading all the topics I could find about batteries I find myself more confused than when I started.
So I have a simple question, and I hope someone can explain it to me:
Regardless of the Li-Ion vs NiMh debate, is there an easy way to decide whether the 4.8v packs or the 7.2v packs are better suited to certain applications? e.g. is there a situation where, say, 7.2v would be too much or 4.8v not enough?

I apologise, but I really am a novice at electronics.

[xwingband]

If you are resistoring a saber... less is better. The excess voltage is going to be taken off as heat so you want to keep it as close as possible to the Vf of the LED.

If you are driving it... more is better. A driver can take advantage of the extra voltage and it doesn't get wasted. So as long as you can fit it more voltage is fine.

[Angelus Lupus]

So when using a driver the excess voltage equals greater runtime?

Thanks x-wing, I obviously needed it put that simply, but now I understand.

[xwingband]

Yup, that's essentially it. It's like overdrive on your car.