Stainless strength is ~1600MPA at -200C, Titanium is ~900MPA. Density difference is 8.0g/cc vs. 4.5g/cc for titanium. So at low temp, strength to weight ratio is about even.

Maximum working temp for stainless is ~800C vs ~600C for Titanium, therefore requiring a smaller heat shield. Some parts of the vehicle will require no shielding or insulation.

So strength to weight is a wash at low temp (fully loaded with propellant), stainless is better at high temp (re-entry) and Titanium costs ~10x more. Aluminum is also cheap and easy to work with but will require larger heat shields. SpaceX is making a trade-off between weight, re-usability, development cost, and development speed. I really think between the marginal performance benefits and significant manufacturability challenges Titanium just doesn't bring a lot to the table.