The way a bike ins constructed has a significant difference in how it rides. I have ridden some aluminum frames and there is no way around them being more harsh than other bikes in general. Now I have a late 90's Trek Police bike and its one of the best riding bikes I have ever ridden very smooth ridding bike. The mini velos I have are not harsh either and they are Aluminum bikes. I've ridden some steel bikes with decent to relatively good tubing and they weren't as smooth,but a little more zippy I think. There are so many factors when it comes to a bike frame its hard to really ay one way or another without ridding it. Some frames just feel right for one person and not as much for others. I rode a beat up Colangelo someone was selling for a good price and I had to admit it was a very nice riding frame, but I passed becasue I was more than I wanted to spend even though it was pretty good price.
The bottom line is get a bike you like and ride it.