I would add comfort as a criteria as well.
I have two ventile smocks of two grades.
The lighter one i think is the L24, and that is really comfy - flexible and water resistant. As a double layer in the right spots it would be superb.
The heavier one i think maybe the L19 or heavier. When this gets wet, it really stiffens up, and i would say is more uncomfortable.
All this assumes, of course, that i am right in which grade is on which smock.
Either way the standard grades seem to be very hard wearing, and my lighter smock is the preferred one. Used with a good synthetic layer it's very good.
The talbot site suggests that the best all round grade is the L24, so that is the one i went for. I will be doubling on the shoulder and elbow areas for good measure though.
Interestingly, or not, is that none of the manufacturers actually say which grade is used on which article....