Something to consider is what type of diving you will be doing. If you're boat diving at depth you definately want the bigger tanks. If you shore dive like a lot of us do, with long walks to the water over rough terrian at times, you might want a lighter tank. I have 2 steel 100's and a steel 72. I find myself using the 72 a lot more than the 100's. If I am in the shallows less than 30' - I can get almost an hour out of it. I was in for over an hour yesterday playing underwater tourist in 20' depths and only used up 1/2 of my streel 100. It also depends upon what kind of shape you're in. If you're in your 20's, then toting a 120 around might be a piece of cake. If you're in your 60's like me, I'd break an ankle on our slippery rocks trying to lug one of those on my back. I seem to do a lot of shallow, one tank dives and the 72 suits most of my dives. So if you know someone with a boat that goes out a lot get the big tanks. If you just want to play in the shallows, consider a smaller one. But get steel - the less lead you have to carry - the better.