Get one of the 25 liter packs. As far as the weight goes. I was surprised at how little the system weighs in an unloaded pack. Not much. Plus the Mammut Packs carry very well. At least the 30 liter does. They do weigh a whole lot less than the BCA versions for certain.
It is more weight, no way to get around it. The extra layer of insurance is well worth it imo.