Can't say I've noticed all that much difference (though the only trike I've ridden a lot was a Trice QNT which had a steel main section and aluminium extremities). On recumbent bikes I'd ridden a Streetmachine (steel), Grasshopper (aluminium) and both a Giro 20 & 26 (both steel) and to be honest I think the design of the seat and presence/absence of suspension make far more difference than the choice of frame material.

That said, it would be interesting to line up a Giro 26, Corsa, Ti Aero and Carbon Aero (with suitable quantities of duct tape to disguise things) and see if people really could tell the difference!