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The only advantage that I see of using a whole horizon mirror in conjuntion with a 7x scope is being able to use the double prism filter found on some sextants like the Cassen's and Plath Horizon Ultra.  Since the prism can only be used with a whole horizon mirror, it is a necessary feature.  Other than this, a split mirror with the 7x scope gives the advantages of better light transmission along with the apparent whole horizon view which I find easier to use from the deck of a ship.

Of course if you are swapping scopes on a regular basis, you will lose the whole horizon view, so maybe you want that whole horizon mirror on there for low magnification work.  I think the selection of features depends greatly on what you are shooting and from what platform.  I don't think that there is a universal configuration for sextants which is why  I chose a sextant that gave me numerous options.

Jeremy