Sand is best if you think debris will get trapped on top of the bed of sand and you’ll occasionally want to do a sand change (every 3-5 years is about right). Glass – you should get a cleaner backwash and not need to change the sand – however, if debris still gets deeper into the glass, I’d assume you’ll need to change out at least some. We are installing sand when customers want a simple sand change, and glass if they want to pay a bit extra and see if it eliminates the need for future sand changes. I currently have glass in my sand filter. We use Dr. Dryden by Maytronics.