Ali… harsher ride, with bumps less easily absorbed by frame, but more direct and much siffer ride. Also lighter for the same strength/stronger for same weight.
Steel, more compliant ride, with less trail shock transfered to the rider, feels softer and hence more flexy. Can be a nice lively feel from a good steel frame, or can be just flexy and twisty from a bad one.
steel you can repair, and ali just goes bang very quickly.
Depends on how the frames are built, a good well built steel frame will have a bit of a zing to it, a badly built frame with rubbish tubing can feel a bit solid and dead.
Aluminium can be really stiff and harsh but with teardrop rear stays and the like can soften it up a bit.
they will both feel harsh on the metalised roads around rivvy and not massively noticeable IMHO alloy does not rust when your paint is chipped so that is a plus for me. quick view here http://www1.epinions.com/content_3704660100