RE: Two 5s packs or 1- 10s pack
Every 10s pack I've seen has two 5s balancing plugs. Revolectrix Cellpro 10s/ 10xp, Turnigy 10xc and the iCharger 1010b+ are examples of chargers that will charge 10s packs just fine.
I use two 5s packs because failure of 1 cell only requires replacement of 1 pack.