TSX is stiffer, but not necessarily better, especially for a 150 - 160 pound rider. SL, SP, SLX, SPX, and TSX are all the same metallurgy (Cyclex), so the quality of the alloy is the same; they're just designed for different purposes (different riding styles, different frame sizes, riders of different weights and strength, etc.).

Titanium is a great frame material and isn't density challenged like steel. It also doesn't corrode and can be left unpainted. If you want to put a steel fork on a Ti frame, there's no reason not to.