One thing you MUST do is allow the glass heater to acclimate after a water change before plugging it in. If you don't, it will stay on; forever it seems.
I have quite a few glass heaters, and quite a few stealth heaters
The stealth heaters are always right on the money. I set it for 80 and my digital thermometer says 80. No worries.
Glass on the other hand, it varies 2-3 degress either way. I have made Cichlid Soup before but that was my mistake.
Moral is, if you can afford Stealth Heaters, by all means get them.
If you get glass, follow the directions and monitor it for the first few hours. After that, you should be golden.