Ortlieb, but I'd just get the pannier , and use a rack,
then add the Back pack harness when you wanted to carry one of them,
I dont like backpacks on long trips at all
but they also make a messenger bag
and you can get a portage pack . cascade designs , seattle sports , lots of similar makers
Canoeists Portage between streams and lakes , the dry bag gets carried
and the canoe is held overhead, supported on the Shoulder thwart, which is at the balance point.
You can also tie a Portage pack in a bike trailer,
and back pack the trailer and all when needed.
how do you want to use the backpack? If it's something carried on your bike to be used as needed for a backpack just get an ultralight drybag to line it with. If it's something you're going to be wearing I'd get one of those low volume very narrow backpacks where half the space is taken with a water bladder and do the same thing with an ultralight dry bag.
I actually tour with an Osprey Sirrus, which I've been very happy with. I'm pretty sure that's the model anyway. The nice thing about the Osprey packs is they come with an integrated waterproof pack cover--super convenient. I hate riding with anything on my back, so I actually attach the pack to the back rack using some small velcro luggage straps. This is easy to do on the Osprey pack because they have an exposed frame in the backpanel.
Here's a picture of my gf riding with the same setup: Untitled by bencblue, on Flickr
This isn't the lightest setup, but I think the situation where it's really useful is during city visits where it's just nice to be able to carry a backpack if you want to.