If you are ordering them from Hobbyking, i'd rather take the Rhino 4900. I think the regular Turnigy batteries are much heavier.
http://www.hobbyking.com/hobbyking/s...idproduct=7654

I got 6x5s packs last year and again 4 packs in the spring. The old ones have better internal resistance (~10-15 mohms for the whole 5s pack) but had 1 cell that failed.
The new ones have higher IR (~25-30 mohms) but these have far less cycles on them. Except that 1 failed cell, all the other packs are going strong.

Turnigy Nanotechs seem to be good batteries too, but again for the same capacity they are heavier. If 4500 mAh is enough for you, you can find 25C 4500mAh Nanotechs that are very light.