There is absolutely a difference. Since glass was introduced we've seen thousands of new products introduced. Thin stuff, thick stuff, rounded edges, non-rounded edges. The truth is tempered glass can absorb impact and save a screen from breaking after taking a fall - something a regular PET screen protector cannot.
I've said this numerous times - it really comes down to what you want the glass to do. If it's just protect the phone from scratches without getting the screen protector too scratched up in the process, any glass screen protector will do.
However if you are one of the people looking to really protect your device and not just your screen if definitely makes a difference. Two things to me matter. Thickness and edges. In independent testing we found as little as .08 mm difference in thickness means a whole lot in terms of impact protection. (Full disclosure I work for BodyGuardz). .4 mm thickness nearly doubled frontal impact protection as compared to .32 mm (Average break height of 86 cm vs 46 cm).
The popular thing right now is a rounded edge. This looks awesome. But the process for creating a rounded edge involves grinding and polishing which creates micro fractures and weakens the glass and reduces the amount of impact it can receive. Again, it comes down to what your asking the screen protector to do. The rounded edge is also present around the home button.
It comes down to what you want. And while the price point is higher than plastic screen protection, the margins are smaller than you'd think for the good stuff. The cost to get quality glass isn't cheap.
Hope this helps.