For the inside of my tank, I use the magnetic scrubbers, like that great white you mentioned. You just need to make sure you get one with a strong enough magnet for the thickness of your acrylic. Once I am done with the magnetic scrubber, I use a hand held one to clean the top edge and the bottom near the substrate. For the outside, I use plexis and a nice clean soft cotton rag.
The only way you will scratch your tanks inside with the magnetic scrubber is if you get some crushed coral or coraline between the pad and the acrylic. Just be careful and pay attention. If you see something in the pad, remove the scrubber and clean it before proceeding. Scratches can and will happen, even if you are careful but there is really no other way to clean the inside of the tank.