Flare nut wrenches - your preferred design?

I am designing some six-point flare nut wrenches. One of the goals is to provide a wrench that is the least likely to round-off the corners on soft flare nuts. I would like to know experienced mechanics' (not necessarily professional) preferences on a couple design details.
1. Design detail #1 has to do with WHERE the slot is positioned. There's two choices.
a. The slot is centered on one of the six corners of the hexagon, so major portions of two of the six sides are removed. This is the design currently produced by most major tool makers.
b. The slot is centered on the middle of one of the six sides of the hexagon, with that entire side removed, plus minor portions of the two adjacent sides. This design was common until 10 years ago, now mostly replaced by design '"a."
2. Design detail #2 is whether the wrench is flank drive or not.
If you have used both types, what is your preference?

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