Sapphire crystal glass is more desirable because of its scratch resistance. Although sapphire is less shatter resistant than mineral glass, this is not very relevant unless you expect extremely strong impact on the watch (e.g. for military use). Scratch resistance matters though: through day-to-day use you'll probably swipe a wall with your watch or at some point even drop it on the ground.
With a mineral glass you'll get unsightly scratch marks that you'll be reminded of every time you glance at your watch. Sure, you can replace mineral glass if it gets scratched -- and the watch is designed to be repairable -- but it's a pain. And if you're investing in a watch, you may well get a good piece of sapphire that will look good as new for years and years.
Sapphire crystal is much more expensive than mineral glass. Brands that are really serious about quality only use sapphire crystal.
In our opinion it’s practically sinful to make watches with cheap mineral glass -- a few dollars extra in production cost makes the difference between a disposable, high-maintenance accessory and one that will look great for decades.
3/ Check the movement
The movement is the engine of the watch that drives the hands around the dial and powers the watch's other functions (e.g. calendar, chronograph function, etc.). Movements are not made equal. A good movement will keep team reliably, a badly made one will not.
[PIC OF AN INTERESTING LOOKING MOVEMENT]
Swiss and Japanese movement makers have the best reputations whether it’s for automatic or quartz movements. They lead the industry in innovation. Some of the most well-known Swiss movement makers are Ronda (particularly for quartz movements), ETA and Sellita. In Japan you have Seiko, Miyota and Citizen, etc. There's also a number of independent movement makers, particularly in Germany and Switzerland.
However, as with any "Made in" designation, it's impossible to say, "all movements from [country] are good". Any country has good suppliers, not-so-good suppliers, and downright bad suppliers. Best is to ask who the manufacturer is for the particular model and do some searching for yourself online.