Aluminium cylinders get floaty bottoms during the dive and must be trimmed out. Steel cylinders stay negative for all the dive. If you don't have to negotiate restrictions, steels are quite comfortable - just clip them on and forget about them. But if you have to unclip the bottoms and bring them in front to get through a restriction, steel tanks will sink and make it difficult to maintain your trim. Thus, when diving with AL cylinders, you need a rig which allows to trim out the floaty bottoms. Xdeep Stealth 2.0 Tec arguably is the best SM system on the market right now, but Apeks, Razor and Diamond can handle AL tanks, too, with their sliding rings on the waist band. As far I've observed, it is more complicated with such systems as Hollis. For steel cylinders the important thing will be the lift capacity.