This might be dumb but ... I've heard that steel tanks are preferred because they stay negatively bouyant from full to empty and aluminum changes from negative to positive. A Catilina 80 CF aluminum goes from -1.8 to +4 while a Worthington 80 CF steel goes from -9 to -3. In each case the difference is (about) 6 pounds more bouyant when empty. What is the difference? Physics & common sense tell me that 6 pounds is 6 pounds. What am I missing?

The positive buoyancy at the end of the dive using an aluminum cylinder has to be countered with lead to help maintain the diver's over all buoyancy. If you are constantly trying to float through, say a safety stop, you'll find it tiring, irritating and can actually give you a bit of a headache (that will go away after a bit) due to build up of gasses in your system.

With the steel cylinder, that is basically 6lbs of lead that you don't have place into your weight belt or weight pockets of your bcd. Also, that weight that you gain from using the steel cylinder is spead along the lenght of your torso, not only being more comfortable, but also making it easier to maintain your control underwater through your dive.