Without doubt the polycarbonate sextant is the one to choose. I began sailing with a metal sextant. They are very heavy, especially when you are on a moving deck, and having to loop one arm around a cap shroud while you try to bring the sun down to the horizon, and check your chronometer with the other.
The Davis MK25 is an excellent and accurate machine. Treat it like it's precious and it will provide accurate readings and is lightweight.
Many will tell you that a sextant belongs in a museum along with chariots and horse whips, but it is a wonderful skill to have, it is practical, and there is nothing quite so good with which to observe an eclipse than a sextant with all the filters across.
There is an excellent book available (I can't remember the name of the author), called Celestial Navigation In The GPS Age, or something very similar. It's worth buying. These days, there is no real need to use sight reduction volumes. Apps and calculators can do the work for you...but still, there is nothing quite so satisfying as taking a noon sight, then working out with reasonable accuracy, where on Earth you are.
And, used on it's side, like a pelorus, the sextant makes triangulating a plot off the coast, using three identifiable marks, a breeze.
Finally, using a sextant marks its user as a mariner, not just a sailor with a knowledge of GPS, but a mariner, a seaman, a salt...one of the chosen few. Enjoy.
"if at first you don't succeed....Redefine success"!