Get yourself a good 2/3A 1500mAH 5-cell NiMH pack and you'll never have any problems with voltage sag. Those packs will deliver 20A continuously without complaining!

But the battery of choice has to be A123. 6.2V with an incredible current capability and a 15-minute recharge (from flat) capability.

I've switched to A123/LiFePO4 in all my planes now (even my slabs) and would never go back to other chemistries. I can grab any plane in my hangar and go fly it without worrying if the battery has discharged over winter because A123/LiFePO4 packs also have a very, low rate of self-discharge.

The A123/LiFePO4s offer all the benefits of LiPo but with the following benefits:

1. they don't explode or catch on fire if you abuse them or something goes wrong
2. you can run them flat without damaging them
3. recharge times are around 15 minutes from flat
4. they have a low-enough voltage that you don't need a regulator for a 2-cell pack
5. they're cheaper than a LiPo/Regulator setup

What's not to like?