My understanding from the people from DROPRO is that the magnetic scales are the latest thing. They can be trimmed to any length to fit any machine, they are not susceptible to chips, grease, oil. They actually have one working in their fish tank in their office which has been functioning for over a year. Now the glass is old technology and is cheaper. It cannot be trimmed to specific lengths and must be ordered to fit and they do give you excellence assistance in getting the lengths right when you order. The glass are susceptible chips and other debris. You cannot use an air hose to blow the chips off of your machine. If they stop working you can send them in for repair but if they are contaminated chances are you will be replacing them. They told me that if I took reasonable care and did not let other operators use the machine without supervision, they would last a long time. I got a three year warranty with mine.
So it you are thinking on putting the machine in a production environment I would go for the magnetic and avoid the possibility of downtime. For me, as a hobby machinist, the glass will work fine.