Ok, well stainless steel retains heat heat better, and radiates it a bit less, has a higher heat handling capacity. It also looks better than a cast.
As for the tial vs garret housing, in the size you are looking at: the finish of the tial housings is a lot better inside and out, so it looks a lot better in the engine bay, and the inside surface is a bit smoother. The tial housing weighs a bit less, as they can afford to case them a bit thinner because they are a made from stainless. Depending where you buy it from, the tial housing will be cheaper bu a fraction. And finally, garretts v band housing is a .61, tials is a .63.
Garretts are usually sold with the vbamd clamps, but no flanges and tials are usually sold with no clamps or flanges. The only reason i would recommend a garrett to someone over the tial is that garrett make twin scroll vbands, and tial only make songle scroll.