i've had both steel and alum frames. i think i prefer the snappiness of an alum frame generally, but the alum frames are definitely harsher. i had a niner MCR and it was definitely a "warm" ride, which was nice on those longer distance rides, while my other x/c oriented hardtail (specialized aluminum hardtail) i had at the time was harsher, but i felt faster on it. i don't think i was any quicker, i just felt faster on it tho.
now i'm on a aluminum hardtail that's heavy as a pile of rocks. its not snappy, but it feels like a freaggin' tank. so i like it.