i was going to order 3-4 of those (40c) packs, until i heard the turnigy 6s packs have a tendency to puff easily... not sure how true, but now i'm accumulating flightpower and thunder power 6s packs. i just don't want to take a chance.
I've seen a few reports of bad 5s turnigy packs, but the flightmax packs fair better it seems. As for the price difference... weird, but the 35c pack does seem to be slightly larger than the 40c pack going by the listed dimensions. I'd go with the flightmax of the same spec ( 35-40c, its all gravy ).