Definitely SS.
1. The molecule structure of steel is more uniformed than aluminum so forces (recoil) are going to be distributed more uniformly.
2. The steel is twice as strong as aluminum and you need up to two times less space for steel design. Design does not have to be compromised for additional space like with aluminum.
3. Aluminum reacts with steel when is in contact with it for longer and looses its structure and mass (similar to corrosion). I’m sure designers have the way to avoid this but when there is steel-steel combination it is going to have no issues for sure.
4. The most important – aluminum heats up much faster and melts at much lower temperature – in most cases we are not going to see the difference but sometimes there are extreme conditions (fire exposure, long lasting exchange) when this can be crucial factor.
See table:

Metal: Steel Thermal Conductivity: 52; Melting Point: 1500(°C)
Metal: Aluminum Thermal Conductivity: 190; Melting Point: 620 (°C)

I see no points to support aluminum – maybe cost, but I think choosing SS over Alum. is well spent money.

I would have to say that I would go with the SS as well because if I have to use it I want it to be as accurate as possible. I do not want to have to shoot twice to get t he job done if my life was to depend on it.

An alloy P226 in .357SIG is my daily carry, and I like the lighter weight. However, in general I prefer the steel frames, and would probably trade for one if the opportunity was there. Boils down to personal preference...