About what rcmodel said. Unless your scope tube is steel, I think aluminum makes more sense. There's also the coefficient of thermal expansion - if you use aluminum rings on an aluminum tube (which is 99%+ of modern scopes) they will match. Use steel and they won't.
I will happily use steel when good cheap rings are made from it - Warne or the Leupold QRW/PRW. Anything else, usually better, I buy aluminum. The amount of clamping force intended for scope rings is far, far, far below the level at which extra strength matters, and to compound things common aluminum for scope rings is 6061 or 7075, both of which are strong, while common steels are the simple carbon steels with little or no strength benefit over those grades of aluminum. (I have seen one or two brands of steel rings advertised as 4140, which potentially is much stronger - these were $200+ rings intended for 2lb+ scopes on 50 BMG rifles.)
Good aluminum is not easy to strip out. For reference, I've all but destroyed the threads on a STEEL machine screw going into a 7075 ALUMINUM AR lower. I was surprised and impressed. The lower's threads were fine despite having essentially cold forged the end of the steel screw. (Screw was longer than spec, although it worked in most lowers - lower was maybe out of spec, hole for the grip screw wasn't threaded for the last .2" or so. I think the makers of each took a shortcut based on common practices, and their shortcuts collided.)