Well there is no doubt that steel is better, as long is its not one of the powdered metal or cast rails we are seeing on the market now from Thompson, Buris, Weaver and all the other rebranderd china mart stuff
If you are looking for a quality base that does not cost over $100, then is will have to be aluminum.
First off, there is eveything from cheap exstruded weaver style to fully machined barstock 6061 T6 Aluminum.
The issue i have with exstruded or cast aluminum is that you se ealot of change in the spec, sure some are machine finished, but when you see companies like EGW selling off lenghts of exstruded out of spec railfor cheapmakes you wonder....
Also to be talked about is the finish.
You will see eveything from powder coated raw aluminum , to mil spec type III hard anodized.
You need a good tough smooth finish,many such as millett ( chinamart) acid dip then typeII anodize so it looks close to mil spec...
If your going to buy a good base that does not cost alot you should be looking for 4 things.
Machined for bar stock
Bead blasted not acid dip
Mil spec Type III Matte Hard coat anodizing ( or better)
bbb