It's not just the size that counts.
The numbers you should care about are delivery time, BTC/GH, and W/GH. Technical details are interesting, but if they don't improve those three numbers they're not really relevant.
The correct unit is J/GH you may also say W/(GH/s)
maybe thats the correct unit of measurement but it is shit to say because you dont pay joule on your Energy bills. the pricing is in kwh.