Michael,
I believe the deal is all screens except the "s" can show dof for 2.8 and greater due to how coarse the ground glass/screen is.
So a 3.5 T/S would be fine with all the screens.

The reason they made the "S" line of screens was for faster than 2.8.
It supposedly shows the smaller dof that say a 1.4 or f/2 lens is capable of.