Titanium is much more expensive.
Titanium is less dense and less strong per unit volume than steel. If you simply replaced an existing steel part with a titanium part with the same dimensions, it would be less strong. The advantage comes in if you can make a part of different dimensions. Then you can exploit titaniums lower density.
There are two things that affect the strength of a part: the properties of the metal itself, such as the ultimate tensile strength and modulous of elasticity, and the dimensions or layout of the part. An I-beam would be stronger along its long axis than a square beam made from the same metal.
Classic example: bicycle frame tubing. If you replaced an existing steel tube with titanium you'd have a less strong, more elastic tube. If you make the tube bigger around you can increase the radius of gyration, which means you'll get a stronger, less flexible tube, while keeping the weight the same, because Ti is less dense.
Some steel parts can be simply replaced with ti because the parts aren't very heavily stressed--for example, AR-15 firing pins. Ti is plenty strong enough to work with the same dimensions, and the lighter weight can help prevent slamfires.