I've got both types but use the 2 cell ETec 1200 packs in my Aerohawk which gives me around 20 plus mins. I prefer them because they are lighter than the Kokam 1500 HC pack.
Has anyone tried the older Kokam 1020 cells? They are lighter still but don't have the grunt being rated at a max of around 5C to 6C.
I also tried a 2p2s ETec 1200 pack (giving 2400 at 7.4v) but I found the extra weight just a little too much. It flew OK but I normally hover around half throttle and with the 2p2s 1200 pack it needed 2/3 to ¾ throttle.