Stainless is harder than regular steel, including 531 so you can make the walls thinner and therefore lighter. This comes at a price- IME stainless is often more brittle. It is certainly harder to work as anyone who has tried to drill it knows.

A frame made with stainless weighs about the same as TI, but should be less whippy. I hear stainless rides just like good 531 steel, but simply weighs less. I should know soon- just got a stainless frame. Its really pretty... but I'm still building it up.