Currently, this is how I generally handle it.
Total base + water < 12 lbs = GG Murmur
Total base + water < 20 lbs = ULA Conduit
Total base + water < 30 lbs = ULA Ohm
Total base + water > 30 lbs = Dueter 42 pro, Gregory Whitney 95 or Kelty Serac (external frame) - none of these are UL packs.
Actually, with these heavier loads of water, I am getting less thrilled with the ventilated Dueter. The mesh panel system moves the center of gravity backwards, which is probably similar to your vented pack. With a lot of water, lets say 2 gallons or more, the old Kelty still kicks butt.
With loads over 30lbs, I will always go for a "heavy" internal or external pack, because they handle the job.
Here is the difficulty... if I can get water during the hike, then a lighter pack gets me there faster. This is the tricky part, getting the right balance between weight and mileage.
Now if I am hiking for several days with no reliable water source, then I have no choice but to carry large amounts of water with a "heavy" pack.
But if I just need a lot of water for one or two days, and I will be consuming water as I go, then I may take a lighter pack, because it will get lighter by the hour.
The other factor is the time of year. For me, up to 100F is pretty easy hiking. I do hike in triple digits, unlike most people. Of course this requires more water.
So I guess the bottom line to all this rambling is that I have several packs for different trips. Oh, and this is not a problem, because my wife has many, many more pairs of shoes, than my total number of packs, stoves, and shoes!! :)