Had a Tourmaster 18L strap-on that had what must have been a quarter inch thick pad. Strapped to a 6.5 gal Trident tank. No marks at all. After letting it go with the bike when I sold it, I just had to have another. The bag strapped to the BA works great. No marks on the tank... yet.
Never heard anything good about magnetics. I'd be paranoid about slapping it on the tank and denting it, and having it screw up my digital camera and anything else I'd put in it. If they're powerful enough to keep it attached to the bike, they must be powerful enough to wipe out e-stuff.
