The main properties of aluminum relative to steel are:
Aluminum has 2-3 times the thermal expansion.
Not always. It will depend on the location Rails aren't subject to thermal expansion unless it's a full auto action where the heat from ignition heats the receiver....
Aluminum has about 1/2 the weight.
Fair Statement
Aluminum usually has less strength.
Not always true. Aluminum comes in many alloys and many are inherently stronger and less corrosion resistant than steel counterparts.
Aluminum can corrode, steel can rust.
Fair Statement, however both can be treated, aluminun anodized and steel parkerized
Both can be cast or machined.
99% of aluminum that's used in intrinsic assemblies (like a rail) is extruded, never cast. The steel starts out (in this case) as flat rolled sheet, the only casting is the slab prior to rolling
Both can be high or low quality.
Yes and no. Machineable steel and aluminum must exhibit certain qualities to be machineable and a picatinny rail made to MIL Spec must be a predetermined alloy.
I usually choose whichever material the base attaches to. That is, I put steel bases on a steel receiver and aluminum bases on an aluminum receiver, such as an AR-15 if they have a large area of contact. If the rail bridges a large gap over the ejection port and won't be as hot as the receiver I may use an aluminum base on a steel receiver if the scope also has an aluminum tube. That can reduce bending from thermal expansion. In any case the most important feature is whether the base is accurately machined and fits both the receiver and the rings.
I do use some of the aluminum optics planet rails, the one made by EGW. They are well machined.
As with many shooting related decisions one parameter gets traded for another. It's not a matter of best, only what's appropriate for a given use.