You need to be careful when making comparisons. As the previous poster noted, same tank ... different volume rating. That's because Faber and OMS use different algorithms for calculating the volume ("voodoo math" for marketing purposes). In reality, at the same pressure, you've got the same amount of air in both tanks, regardless of what's stamped on the crown.
As to the buoyancy, some distributors (e.g. OMS or ScubaPro) rate the buoyancy with a valve, while others (e.g. Faber & PST) don't. Not that anyone would ever dive a tank without a valve, mind you. The reason the former does this is because although the tanks may be the same, they use their own valves on the tanks they sell.
So making direct comparisons can be misleading.
BTW - I'm not aware of any steel tanks that are positively buoyant when empty. My Faber LP95s, with valve, are approximately 2.3 lbs negative when empty. My MP72's, when empty, are about 5.5 lbs negative.
... Bob (Grateful Diver)