The way to go is generally (oversimplifying a bit)
1. Get the largest capacity (mAh) battery you can fit into your setup...that's going to be the deciding factor unless you change parts of your setup.
2. Large cells provide power longer than small cells (i.e. SubC vs. 2/3A). LiPo's are a bit odd to compare to other battery chemistries when comparing cell size.
3. Non-memory cells/chemistries are more convenient (i.e. NiMh, LiPo, LiFe)
4. Go with the lowest voltage that will result in your desired performance...it's different for every setup
So...if you've got one of those crane stocks that only fit mini (2/3A) NiMh battery...you're typically relegated to a 9.6V 1400-2000mAh NiMh battery. They're just fine for most setups but will suffer for applications where 1) there's a lot of full auto sustained fire...the small cells struggle to keep up the supply 2) there's a lot of shooting over a long day...the small cells just don't have that kind of staying power when compared to larger batteries (because they're not as efficient).
Of course there's 8.4v mini NiMh batteries that will fit too...if the mAh's the same, what you'll notice is that there's more trigger lag and slower ROF compared to a 9.6v of the same type/rating. All other aspects (vs. larger packs) will be the same. But I'd take a quality 8.4 pack over a no-name cheapo 9.6v pack.
Old simple solution...have spare batteries charged up and ready to swap in.
Again...that said either may work just fine in your setup. If you're taking 20 shots the whole day neigther is going to hold you up. If your rifle is shooting 300fps...no problems.
The "only" other option you have is to go to a LiPo pack...because its form factor is very small (relative to other batteries). In that case you're looking at a 7.4 or 11.1v packs. Again...#1, it's got to fit your setup...or you have to change your setup. What you'll find is that with the 7.4v packs you'll typically get larger mAh sizes than with 11.1v packs of the same physical size. That's because each of the 7.4v packs' cells (2) are bigger than the cell of a 11.1v pack.
7.4v 2000+ mAh 20C pack....you'd probably have no problem pulling a M120 spring all day (or weekend...again depending on how much you shoot). It'd probably be equivalent to somewhere between a large 8.4 or mini 9.6 so far as ROF goes...but it'd last like the large NiMh pack.
Same as with the NiMh pack...go from 7.4 to 11.1 and you'll have a quicker trigger response and higher ROF. Wear and tear would be accelerated with the 11.1v and 9.6v.
So, if you made it this far.....what to do?
I'd say get the largest 7.4v 2000+ 20C lipo you could fit into your stock and have at it. If it shot well...I'd pick up a spare (always have a spare battery for games).
Last edited by m102404; July 5th, 2012 at 15:08..