You should also think about the contact area between the grinding wheel and workpiece.
In case you are dealing with a large area of contact,
you should probably use a softer grade and coarser grit wheel.
Finer grit and harder grade wheels are more suitable for a smaller area of grinding contact.
There are also wheels that are designed to bear a lot of pressure that created when the wheel and the workpiece are pressed together,
plus, grinding machine horsepower should not be overlooked neither.
A useful guideline would be: if the horsepower is smaller than the diameter of the grinding wheel, then a softer-grade wheel should be used,
and harder-grade wheel should be used in opposite situations.