I think I've gone through about every fret file out there from Stew Mac and
others, and I still go back to my three corner file for most jobs. I'm
probably in the minority there, but it's what I learned with and still seems
to be the most efficient for me. When I do use the diamond files though,
it is quite rare that I use the course grit. It's very agressive and unless you
have a tremendous amount of material to remove you will usually spend
as much time getting rid of its file marks as it would take to do the whole
crowning with a fine file.
If you are going to have just one double sided diamond file, I think it
would be much more useful to have two crown sizes in 300 grit than
having only one size in 150 and 300. The crowns on the diamond files are
only an approximation anyway, and you will generally have to rock the file
from side to side to get an even crown. A medium crown file is often
much flatter than a medium crown fret, and if you try to shape the fret
straight to the shape of the file you will often get a bit of a corner where
the side meets the new crown. I've had several customers bring
instruments to me for re-redressing to address this problem after fret
dresses from another shop in my area.
If you have to choose just one, go with the 300 grit offset / rounded end
file that you mentioned. The dual grit files that they sell are a great idea,
but I really don't like the shape of the crowns. The narrow is good for
medium, medium okay for jumbo, and the wide crown seems almost flat
enough to level inlay strips on a fretless conversion.
The crowns
on the offset file seem a little more agreeable to me, and I find the curved
ends very convienient to work with.