Nelson Rigg makes some GREAT stuff. Very rugged construction, zippers that could survive a nuclear explosion, and very reasonably priced. I found a great deal at www.gforcecycles.com on ther expandable tail bag.
I also got the MG950 magnetic tank bag, which is just as tough, but a little small for longer hauls. I can still fit a spare helmet visor in it though, which is nice if I'm gonna be out after dark.
As far as quality, the Oxford stuff is hard to beat.
I would stay away from the magnetic stuff. Unless you are obsessive about cleaning the tank and the bottom of the bag... you WILL scratch up your tank. I have had several magnetic tank bags, and I have scratched every bike I have owned thinking I would be more carefull each time. And they are heavy as **** to carry around.
I would go with the Oxford hump-pack bag. It is convertable from a tankbag to a tail-pack.
I'v often wondered about magnetic tank accessories scratching the tank. I've seen plenty of non-magnetic bags with neoprene bottoms to protect the tank, but I've yet to see a magnetic one with this material... it seems simple enough to just install a thin piece of neoprene under the magnet. I wonder if I could fashion something to sew over the magnets on my tank bag without compromizing the grip of the magnets.
i have the joe rocket 2 layer tank bag and LOVE it. you can have it small or enourmous. problem is...dunno if it is from the bag or not, but i have a few REALLY deep scratches in my baby now...and im not too happy.