There are many discussions and papers on this topic on the web. Basically, sapphire is the hardest glass and therefore the most scratch-proof; however, I also got a scratch on the old Fenix 3 sapphire, nothing is 100%. The big advantage of Gorilla - apart from the lower price - is that it is much less reflective and offers better contrast, making it much easier to read. I chose Gorilla because of its readability and because you don't buy a watch like this for life, so it can take a scratch. If you do rough sports, sapphire might be more worthwhile.