Don't know about the difference between these two metals with folding bikes, but I can tell you from a road racing background there is, in my experience, a difference. Try kicking in a sprint at top speed or attacking on a climb when you are pushing yourself and your bike to the extreme or jumping out of a fast paced bunch to chase down a breakaway on a Aluminium frame and you can definately feel more flex than on a steel frame. In those situations you become accustomed to the slightest change in frame and tyre behaviour, gear and chain oddities, it becomes a 6th sense.
My current road bike is Alu with a carbon fibre rear, it doesn't flex.