After searching Ebay for each one, the answer could be nothing, or lots of things.
Looks like the T **may** have more Z travel, and a larger spindle, but there are 6040's that are the same.
I see 6040's that are the same as 6040Z's, and 6040's that are the same as 6040T's.
I'd pay more attention to the specs, then the names.
Looks like there basically come with 800w spindles and shorter Z travel, or 1.5Kw spindles with longer Z travel.