Would a steel, instead of an aluminium, plate be reasonable?
I would consider getting another aluminum build plate for the following reasons:


Lightweight. Aluminum is a very lightweight metal, making it suitable for most machines that have injection molded platform arms. This reduces potential sagging of the arms and overall load on the -Z- axis stepper motor.
Conductivity. Referring to this simple Google search for heat conductivity, aluminum is significantly more conductive than steel (205 vs 43 respectively) with copper at about double that of aluminum.
Availability. Aluminum is already a widely used material for 3D printing, so finding one will be relatively easy and probably cheaper than having a steel build plate custom made.


In conclusion, I personally would not recommend using steel for your build plate as aluminum seems to have the most benefits. Yes, steel will be more rigid and durable, but I don't think that these should be variables that are significantly more beneficial over aluminum.