I just finished trying this for a few weeks on my Marathon S. I weigh 170 pounds and tried the stock Heavy spring in the ETA side and just air preload in the Rebound side.
To get the proper sag, I was using 30-35psi (stanchions are 30mm on the Marathon and I'm only at 105mm, so yours would be less pressure). The problem that I had with this setup was too much mid-stroke dive under braking. It seemed that almost all of my travel was used under braking, and so there wasn't any extra available travel to absorb braking bumps. Tried raising oil levels up to the maximum of 45mm, as well as increasing air preload without any luck.
Also, in my quest to eliminate the mid-stroke dive and near bottoming, I tried increasing the air pressure to 35-40psi, which resulted in stiction. The fork became really notchy and the rebound started to clunk (maybe lower oil levels would allow lower pressures and eliminate my problems?).
I'm thinking that the 2005 forks have a much heavier coil spring in the ETA side, which would allow much lower air preloads resulting in far less stiction. There isn't a negative spring to combat this added stiction and dive. A heavier coil would hold the fork up in it's travel during braking too.
I'm back on the dual coil setup withe zero air preload again, and I think that I'll be keeping it that way.