A pen display will either one of two surfaces:
Etched glass - glass that has an etched texture. Learn more here: etched glass
Film - A thin film on plastic that sits on top of the glass. See anti-glare film.
The traditional thinking is that etched glass is better. You will see many "pro" tablets that have etched glass while their "consumer" counterparts have the film.
My view is that it is not so clear that etched glass is better. In some cases films have real advantages. In this document we'll discuss the merits of each.
AMOUNT of Anti-glare sparkle - which is an undesirable effect - is present in both etched glass and films. And the amount is highly variable. It's not possible to say that either choices has less of the sparkle. You have to examine the specific etching or film used for a tablet.
This is not possible. There is no known way to reduce the amount etching on glass.
For films, many tablets that do have films have removable films. And manufacturers even sell replacement films.
This is one flexibility that films have over etching.
You can remove the film entirely
If you scratch the film, you can replace the film.
If you don't like some aspect of the film - maybe it has too much AG sparkle - you can buy a different film that has different properties. For example, Photodon sells films that have different amounts of AG effects. You can even get completely clear film.
NOTE: Some pen displays have very securely attached films, and it may be difficult to remove them.
Scratches in etched glass are permanent
Films are clearly better at this since the films can be replaced.
Plain glass will feel slippery to draw on. Etched glass offers some texture and so too do films - so you will find that your pen grips the surface a bit better.
In my opinion the etched glass has a silghtly stronger texture feeling. The films tend to fill a little different. Both are better than plain glass.