There are a great many pros and cons of all the materials. My preference is steel. I have both and I like the solid feel, and the longgevity of it. An aluminum frame is going to be lighter and thus spunkier, but they don't tend to last as long(5yrs mtb racing as opposed to 20+). On road it is not so dramatic, as well as maybe that long term is not a concern.
As always, do what makes it fun. You dont have to know the metalurgy of the frame to know what feels good. Test many. Enjoy.