It really depends on the individual frame. I have ridden some aluminium bikes that are just as comfortable as steel, some steel bikes so harsh (Ragley/On One) that you could never ride more than an hour or so on.
The best steel bike I have ever ridden has been a Kinesis Decade Virsa. It definitely took some “buzz” out of the trail.
The best aluminium was a Pace RC 325.5.
I guess the most important factor is tyres/tyre pressure.