It's not a 1:1 comparison - there's more than just the glass that's different. First of all, it has cellular/LTE built in, which starts at $499/529 in the aluminum version. So, most would tend to say it's $200 more (since the Stainless Steel version w/sapphire crystal starts at $699). At most, it's $300 more than the GPS only aluminum version. But in addition to the cellular/LTE and the sapphire crystal, it has the stainless steel body, which is another upgrade.
Truly, only you can decide. I was all set to get the Aluminum version but became concerned about the Ion-X Glass. I really hate scratched up watches and worried I'd mess it up terrible. I plan to wear it 24/7 except for showers and charging. I have health issues and need the heart rate sensor and blood oxygen monitoring.
If you don't care much about the screen, want to invest in screen protectors (which most say aren't great on AW, though there are a few that are acceptable and usable - and these generally cost $30+), prefer the lighter weight, or simply don't want to pay the extra, go with the aluminum. If you're worried about scratches or will be upset if you scratch up the Ion-X, and can afford the extra $$, go with the sapphire crystal.