Depends on what you want. Personally I like the 5000mah packs, get two and your good to go for sv27 to run in series. I would stick with 2 5000mah packs for now, run em and if you wanted more runs or running time, grab another 2. But that's me, its really whatever you prefer.

If you grab smaller packs you would need 20c-30c depending on the size of the pack but with larger you can go 15c. Here's how the equation works for determining draw.

If one batt is 5000mah and has 15c draw(constant) then it's 5(5000 is 5 amps)X15=75amps constant draw.
If you get smaller, say 2200 at 20c, then equation would be 2.2X20=44amps which would be borderline for sv27 so you would look for 25c or 30c
so one last one just so you get the hang of it 2200mah batt with 30c equals 2.2X30=66amps draw

Generally they also tell you burst draw too, but make sure your constant draw fits the bill.

I've heard the zippys at uh are pretty good, you could ask for others to post other brands.

Ivan.