Take a look at the flightpower prices for 6s packs on tower hobbies. They are the same price as the 5s packs, great deal, IMO. Plus all of the cells are matched to each other, where 2 seperate packs will not have 6 total matched cells, just 2 pairs of 3 matched cells. I just say this as after a hard run with my trakpower packs, one is always slightly stronger than the other, and requires balancing. I run and charge them in series, BTW.
http://www3.towerhobbies.com/cgi-bin...GHTPOWER++6s++
Most of the higher mah packs are the same price as the 5s packs of the same cells. No idea why, but it makes sense to go with 6s.