A hollow steel tube the same weight and diameter as a solid aluminum shaft is probably stiffer.
I would worry about flex of that 3/8 aluminum plate, but then again maybe your telescope size and weight and your application requirements do not make this critical.
I am not a mechanical engineer, and I have never built a German equatorial mount so you probably want to check these things out for yourself. But I was very impressed by Textereou's chapter on mechanical considerations for German equatorial mounts. I would stick with the conventional wisdom of using the thick, heavy steel parts.