I initially had the Strong version which was far, far too powerful for my 12mm glass, it juddered a lot and was hard to move, especially with the tiny grip!
I then swapped it for a Long version but at the same time bought a Flipper magnet....I haven't used the Tunze magnet since. The flipper is much easier and I think there is less chance of scratching the glass....This is because the magnet is much weaker than the tunze, I've scratched my glass about 6 times with the Tunze because grains get caught on the inside of the blade when "reversing" the magnet. You have to be incredibly careful near the sand edge.
The Flipper glides smoothly over the glass on both the felted side and the blade side. the fact it is floats is brilliant too, the Tunze sinks so damn fast and is a pain to get back on the glass without putting my arm in the tank.
The Flipper is much easier to grip than the tunze.
Being able to quickly flip the Flipper means I can use the felt side for the bulk of the glass then flip to the blade to do the sand line. I normally move the magnet up and down while working my way along the sand edge. I really don't worry half as much about sand getting caught in the blade with the Flipper, its not really strong enough to cause the deep scratches the Tunze does but is plenty strong enough to remove spirorbid worms or stubborn algae...I don't have coralline forming on the glass so cant comment on that yet.
I will say the nano tunze magnet is spot on for smaller tanks or sumps. I use one for my sump and my QT and have no complaints with it.