This one:
I will be making through mortises in oak, or possibly some other hardwood. Some of the stock will be 1.5" thick. I was worried that squaring the corners of the routed mortises was going to be a debilitating chore, but I am encouraged by my first couple of tests with this chisel.
At its "just-from-the-package" sharpness, it makes a nice clean corner. Any imprecision will thus be attributable to my lack of skill rather than the tool. I intend to practice up some.
Next, it's not terribly difficult to plow through the oak. OK, I'm only gouging out a 1/4 x 1/4 corner in a 1/2" mortise, but it sure is easier with this purpose-built tool. My test runs have been on 3/4" scraps, so half the thickness I'll need, but it is very quick. My attempts with standard bevel-edge chisels were much more cumbersome. I think that the bevel on this chisel may be at a steeper angle, besides the corner configuration.
Now I need to learn how to keep it sharp. People recommended diamond hones, but I see at least 5 "colors" (grits). Any tips as to which one(s) I need?