I'd say anything but aluminum... Have one of each, my surly rides like a tank, so I'm not sure I have a real feel for what steel feels like. My carbon bike's great, though I prefer my titanium litespeed lately.... It definitely doesn't ride like aluminum. I think it possibly depends on whether you have 6/4 titanium or 3/2 - the former is probably stiff enough that it could be pretty unforgiving.

The ti just feels "livelier" which is a nice change from the carbon feel... Although, sometimes heading down a hill on a chip seal road, I think that I could just repeatedly ride down those hills if I wanted a vibrator.