I don't have any first hand experience with Al, currently ride Ti (Litespeed Classic).

Titanium is "springy," and supposedly transmits less road vibration, etc., to the rider than Al. I've heard that this has been improved somewhat in Al frames by altering the tubing diameter, etc. Al frames cost less than Ti. Ti is not prone to corrosion.

Al supposedly suffers from fatigue, which ultimately causes the frame to break, since it is not a resilient metal like steel or Ti. However, I don't know of anyone that has had a frame break after years of normal use. Every once in a while, every company lets a bad frame slip past their QC department, no matter what the material.

Several years ago Scot Nicol wrote an excellent article for Velonews, "Metallurgy for cyclists". There's a copy still online at:
http://www.sjsu.edu/orgs/asmtms/artcle/articl.htm