There are several metal choices other than W&P. The Astra has developed a good reputation. Also check e-bay. However you have to wade through a lot of cheap reproductions. The WW II vintage British sextants are good choices.

My experience has been that the cheap plastic(also expensive plastic) do a fair job, but their error will change with different grip pressure. Also they need adjustment every time they are removed from the box. Learning and back up OK, but wouldn't want to use regularly at sea.

Accuracy at sea from a sailboat 3 - 5 miles is normal. The instrument is capable of much better, but the platform makes precision difficult. That said the better the instrument, the better the end result - to a point. It is all similar to pistol shooting practice, practice, practice, practice. Also I would not take a Gummint issue .45 to shoot in a match.

http://www.celestaire.com/Marine-Sextants/View-all-products.html