Lots of good reading here.

In general though - 953 is stainless steel, you can probably forget that. I own steel bikes in 725 and 853 and they perform equally (as well as 953). Touring bikes are sometimes lugged so you see the 500, 600 and 700 series being used, although I believe one can build a lugged 853 frame.

The tube specifications themselves matter very slightly in the performance of a bike. What matters is the geometry and overall design. Those are the areas you should be focusing on in choosing a touring (or road for that matter) bike. If you're going with a rack bike, tubing selection is off the table. If you're going custom, have this discussion with your builder of choice.