A new Tamaya would cost considerably more than a new Astra, and so I would expect a good used Astra to cost less than a good used Tamaya. Also I think the Tamara Venus is no longer made, so parts
will become harder to find in the future than for the Astra, which has a large user base and robust sales, and will probably be made for many years to come.
I would expect the Tamara to be slightly more accurate, but the difference will be far less than user error, when shooting from a bobbing sailboat.
So, I would go for the Astra, but insist on a price
significantly less than the Tamaya price
. I actually don't like the Venus model much, myself, preferring a full size instrument. And there are lots of accessories for the Astra that might be cool to have.
A Davis makes an excellent backup sextant, And it can indeed be your primary or only sextant, but unless you are on a tight budget
I think you will be happier with a metal sextant, even a used one. And you can always resell a used sextant for at or nearly the price you paid. The depreciation already happened, before you bought it. So don't be too worried about getting stuck with it should you lose interest.