Other than if you compare a runflat type tire with a standard tire, air pressure will make more difference in ride characteristics than speed rating or treadwear rating.
That being said, there are lots of other competing factors. High speed rated tires don't typically come in high aspect ratio sizes. This means that you won't commonly find a 225/60-16 tire with a Y speed rating, whereas if you drop down to a 50 aspect ratio, there are many 225/50-16 tires that are Y speed rated.
Treadwear also comes from a variety of means. Using the same tread compound, a tire molded with a greater tread depth will have a higher treadwear rating than one with a shallower tread depth. The greater tread depth tire will ride smoother because there's more rubber to absorb road imperfections.
Tires gain high speed ratings by using something known as a cap ply. This is an inelastic band that encircles the tire longitudinally and prevents the tread from bulging out at high speeds. Almost every tire above an H rating will use a cap ply, and many tires above even a T rating will use a cap ply. Depending on the material, a cap ply can create extra ride harshness to varying degrees.
Basically, if you want a tire of certain characteristics, the best bet would probably be to read the Tirerack owner reviews and tire tests, rather than to rely on speed rating or treadwear rating, and to generally stay away from runflat tires if you want a softer ride.
