I'd go 17. Helps you with mortices both regular and through. I think it also gives you a better sense of driving straight down. Because it is longer, the striking end of the chisel must be further off vertical than a shorter chisel to under cut or overcut by the same angle. You'd have a tendency to be a little straighter with the longer. Even so though, you'll get a feel after some practice and get good and driving straight down, 90 degrees to your ref face. You can get pretty close with some experience. At least you'll make sure you don't under cut the line. You can always parr more off, you can't add it back.