I would agree the fatigue properties are largely immaterial unless you are using your tube as a cantilevered suspension device as Bike Friday does. It isn't as though well made frames exercised within their design limits are all that difficult to live with.

My preferred quality is the corrosion resistance. Good for a long range tourer, one less thing to worry about, not that I have many worries about steel.

Some of those who don't like Ti feel that when the material is used to take advantage of the weight saving it ends up too soft for them.

The requirements for welding are pretty much the same as for any material, just less forgiving. And practice isn't cheap. My jig came with a built in purge system and the gas is used with the welder also so nothing special there. I don't weld Ti, too difficult to get up here.