From the web:

Titanium

Titanium tubing is widely viewed as the ultimate frame material. Just as all aluminum and steel are not the same, the quality and properties of titanium can vary greatly with each tube manufacturer.
If you look at the Periodic Table of Elements, you will find Ti below Al., indicating that Aluminum is lighter than Titanium. However, Titanium has superior mechanical qualities over Aluminum. In addition to having an excellent strength to weight ratio, Titanium has a structure which is resistant to fatigue and corrosion.
The simplest and cheapest titanium tubing is known as CP, or commercially pure titanium. This is just what it sounds like; a simple element drawn into the shape of a tube. The typical CP frame is extremely flexible. Although titanium can resist many more stress cycles than aluminum or steel, this excessive flex will quickly lead to a work hardened stress crack. Yes, even titanium can fail.
To improve the ride characteristics of a CP frame, some manufacturers introduce oxygen into the fabrication process. The oxygen embrittles the titanium and provides a degree of stiffness to the frame. The downside is that this process also shortens the fatigue life of the frame.
The best way to improve the property of Titanium is to mix in Aluminum and Vanadium. By adding 3% aluminum and 2.5% vanadium, you get 3-2.5 Titanium alloy.
This next point is important; all 3-2.5 titanium alloy tubing is not the same. How the tubing is produced greatly effects its strength and ride properties. Depending on the method used to draw the seamless tube, minimum yield strength can range from 75,000psi to 105,000psi. Additionally, by cold working the tube, the grain can be altered to better meet specific load requirements. Sandvik is one of the largest and oldest fabricators of titanium tubing. They control the production from the raw ore to the finished tube, and make all their titanium tubing to aerospace standards.

Steel

While the term "steel frame" covers anything from the stamped high tensile rig sold by mass merchandisers to the custom cromoly handbuilt, we'll limit this to the quality end of the range. Steel is appealing because it's durable, easy to work with and repairable.
Basic components of 4130 cromoly steel are Chromium, Molybdenum, Manganese and Carbon. The ratio of elements combined with extrusion techniques is what makes a quality steel tube different from others. If made properly, a 4130 CroMo frame can last a lifetime. Unless a quality steel frame is pushed beyond its maximum yield, it should not fail. Yield strength of "standard" CroMo is @760N\mm2 compared to 3-2.5 Titanium's 792N\mm2.
Steel is durable and can be comfortable and efficient. Springs are made of steel as the material can retain energy and expend it back. A steel frame does not absorb the force of a pedal stroke or the impact of a water bar, it stores it. The gathering of force is what makes a frame comfortable and the return of that energy gives a steel frame a lively feel.