Stainless steel isn't inherently stiffer or less stiff than ti - it's the tubing diameters and build. With two frames of identical build, a SS frame will be stiffer...but ti is usually larger diameter than steel.

Tell your builder what kind of ride you want - maybe tell him about frames you especially liked - he'll take measurements (weight, height, etc.) and a good builder will be able to make a frame that does what you want - ti or steel.