NavList:

I used a whole horizon mirror for 31 years and was perfectly happy with it.  All of my sights were of sun/moon/planets or Sirius (i.e. the brightest star in the sky).  You can do a lot with those.

Then I got to crew on a 45 foot boat sailing from Hawaii to Victoria, Canada, and I tried my hand at a number of star sights.  What I found was that the whole-horizon cut the brightness of objects viewed.  This was true all along, of course.  But I had never particularly noticed it when shooting the sun/moon/Venus, but with any star OTHER than Sirius I discovered that it was indeed an issue.  The week after I got home, I ordered a split-horizon mirror for my Tamaya.

It was a pain in the butt to switch mirrors in the frame, working with the tiny screws and clips.  My fat fingers had a real problem with them.

But having gotten the new mirror in, I really LIKE being able to use pretty much any star I want. Want to try shooting Alnilam, the middle star in Orion's belt?  If you have a split-horizon mirror, then help yourself.  If you have a whole-horizon, good luck.

The other thing was that when using the whole-horizon, particularly if the object was rather higher up in the sky, I spent what felt like a lot of time rocking the instrument, trying to ensure that I was holding it vertically.  With the split horizon mirror, it is rather more obvious if I am not holding the sextant properly...and easier to correct if I am not.

So I agree that it is a personal preference.  But I think that personal preference is going to be driven by the kinds of celestial objects you elect to shoot.

Bob