For MTBs, a steel hardtail does feel very, very different from an aluminum one. Steel feels springy and just absorbs a lot of vibrations. You can feel it in your ankles after a ride. Definitely there is a large difference between steel/titanium and everything else.
I would be very suspicious that some combination of different tires and different geometry between the bikes your comparing is much more responsible for that perceived difference than the different metals. Wearing cushioned running shoes instead of indoor soccer shoes would have a bigger effect i would guess.
