Gloss coat will last a very long time (years) if sealed properly. It comes in a sealed syringe with the screw top. It reacts with moisture so if you use it make sure to remove the exxes air out before you screw the top on (check the 9min mark in the video shown bellow).
More tips in this video
If by any chance the product turnes into a gel form after use then its no longer usable (air wasnt released properly and the coating started reacting and solidified inside).
To answer your second question...both are good. I preper opti seal because I mainly use it as a drying aid and in that regard its fantastic, but you get better water behaviour from Car wax. Durability with both is about the same (6 - 8 weeks).