as far as how 1/4 inch steel will compare to 1/2 inch aluminum in rigidity I do not really know, but I would not use any of the two. On my machine I used Mic6 cast plate for all my aluminum. Mic6 is cast aluminum, which is then machined flat. The stuff is very nice and flat and actually a little bit cheaper than regular aluminum plate. It machines beautifully and does not distort.
You might wonder why it would be cheaper or even the same price, but the bottom line is its cast, so it wont handle high stresses the same way extruded aluminum will. It works very nice with CNC routers though.
I used 3/4 inch plates for my gantry sides on my machine and it is pretty nice and solid. Its thicker than what you are planning but my sides are fairly narrow.