I don't get the difference except from reading what others say. Aluminum is more rigid, steel has some give that makes for a more comfortable ride.
I've had both and couldn't tell any difference, maybe because I'm a light guy or I'm not going over rough terrain? It seems to me that Al 1) doesn't rust and 2) is a lot easier to get on the roof of a car 3) a few dollars more in the budget bikes I'm looking at.
What am I missing?
(I have to add I'm buying a bike for my b'day, mostly to ride 5 mph in the neighborhoods & on trails with my kids, and maybe on 10/20 mile rides for exercise)