NavList:

The 30" of error would still allow  you to find your island so it is definately NOT outside of the range of being "Free from erors for practical navigation."  I have a plastic Davis Mk3 sextant that only reads to 2 arc-minutes, and yet I can usually get a fix to within a couple of miles of my true location so I would be able to find Hawaii with it. It isn't perfectly precise, but it is good enough for "practical navigation." 

Still it is nice to get the best precision possible so havng a certificate that actually states the true instrument eccentricity errors is a plus.  Also the Weems and Plath versions often had the catch-all "Free from error" certificate, and so the Italian Maritime Service certificate is a bit of extrtra provenance.  Personally I would find it a nice addition.

The higher the magnification the harder it is to hold the image steady when on a moving deck. Also the increased magnification usually (but not necessarily always) implies a smaller field of view and a dimmer image. This is why the 6X and 7X monoculars are often called "sun scopes" while the lower 3X and 4X scopes are called "star scopes."  The dimmer image and small field of view are a bit less convenient when taking star sights in the low light of twilight, so the lower power, wide field of view, brighter scope is preferred for those sights. However either scope can be used in either situation.

Still, none of that is a hard and fast rule. Excellent optics outperform mediocre optics every time regardles of the magnification. I have a 2X scope on a Freiberger yacht sextant that is far superior to the 4X scope on a Weems and Plath that I also own. ( Yes its original certificate also has the "Free from errors for pratical navigation" statement)

Another big question is the condition of the optics. If there is any cloudy-ness in any scope caused by age it will seriously effect the performance.

One last comment: Any one who says they "know nothing about sextants" but has it priced it perfectly to market should be approached with a healthy dose of skepticism.  They obviously have learned enough to figure out what it is worth -- if it is in perfect condition -- And yet they can't say that they know it is in perfect condition.  Hmmmm... Will they allow  you to inspect it and return it if it isn't? Or is it buyer beware -- Sold "as is" with no returns?

If the Naval sextant has a star scope along with that extra certificate and is from a seller who is knowledgeable -- that is the preferred sextant.