Correct, they are the same

If you look at the dropdown selection box, the 11x28 is sold by Bike Nashbar, when you change to the 11-28T it's Amazon.

The PG850 is probably a lot lower spec than was on you M2 originally, but getting high end 8 speed has been hard for the last 15 years.

You haven't mentioned changing the chain, hoping you are doing this at the same time as the cassette, as a worn one won't mesh well with the new cassette