Ti has higher mass than Al. But Ti is also much stronger, allowing fabricator to use less Ti material to achieve the same strength as Al. I think if done properly, Ti wheel can be made lighter than Al wheel.

Stainless steel also has higher mass than Al, though I don't know if it is stronger enough to make up for it. Al, being softer, is more susceptible to dent, but repairable by wheel shops. With Ti and stainless steel, owners are shit out of luck.

 

Magnesium wheels are proven lighter and used in prof. races. I'd sport a custom set of that if I was loaded.