Correction and/or clarification are in order:
"Steel bases expand and contract more when the temp...changes than aluminum. Just something to think about."
Not true as stated. It is the other way around. Aluminum has a coefficient of linear expansion of 128 compared to mild steel which has a value of 65 (ignore where the decimal point is for the moment). Aluminum will expand and contract 2 x as much as steel.
With that:
The aluminum base and the steel base have approximately the same cross section at the groove or slot that accepts the weaver style ring. Aluminum has a modulus of elasticity of 10 while steel has a value of 30 (once again ignore the decimal points.) Given the same cross section, steel is 3 x stiffer than aluminum.
I use nothing but steel bases. In my opinion, the 6-screw base is advisable for very heavy recoil calibers. I have the 6-screw base on my 30-06 pistol barrel, on my 500 S&W pistol barrel, and on my 375 H&H rifle barrel. The 4-screw base overhangs in the front which is not a problem with light recoil and/or light weight scopes. I actually have the 4-screw base on my 30-06 rifle barrel and it performs just fine.
I discovered the limitations of the aluminum base when I had mounted a large heavy scope on my 22-250 rifle barrel. I could visually see the front of the scope deflect just with downward finger pressure. The steel base resolved that with just 4-screws. And the rifle shoots 1” groups all day long. In this instance the overhang with the large scope worked out O.K.
I have the 6-screw base on my 30-06 pistol because I was unable to get my scope to stop slipping in the rings. I opted for the J.D. Jones setup with 3-rings which includes the 6-hole base. Since then, if I suspect I will have a concern for slippage, I simply try 3-rings in combination with the 4-screw base. I have 3-rings on the 30-06 rifle barrel mentioned above which has the 4-screw base.
The 375 H&H was something I did not want to monkey with so I sent it to J.D. to have the 6-hole base installed and I am using 4-rings. Not about to go anywhere.
The 500 S&W pistol barrel was another story. I initially tried the 4-screw base. I mounted the base and went to the range. I soon discovered I had been shipped the wrong base. I had mounted a Contender base instead of an Encore base. After I got the blood to stop from the cuts in my forehead and stitches applied, I shipped the barrel to J.D. to get the 6-screw base and 4-rings. I suppose the properly sized 4-screw base would work but I did not want to have another run in with my 4x Burris scope which survived the ordeal. I am finally over the flinching! In the final analysis, a properly fitting steel base is better!
I suppose I should recognize part one of the original question…I use the 3x9 Burris Fullfield II scope on my 30-06 rifle barrel and it works just fine. You can get one at auction for around $150.