Long term use of any magnetic tank bag will cause some finish imperfections however slight. We have an extensive thread somewhere on film patterns to use to cut film for the tanks to use under tanks bags, magnetic or not. I don't use any however.
I have two magnetic tank bags. A large expandable Nelson Rigg and a medium sized, non-expandable Tour Master. I like both of them equally but obviously use them based on what room I need. Both are very well made and come with rain covers.
The Nelson Rigg in side profile. Astute Aussies and Kiwi members will recognize the Ned Kelly pin on the bag. The NR has the less secure mounting of the two because the magnets are on top of the tank vs. down the side like on the TM. It has always ridden behind a windscreen and I've never worried about it coming off. You can place a security strap around the frame in the front if you feel the need for it. I believe some of the piping and trim is hi vis reflective at night. The bag expands significantly if you need extra room should you unzip the bottom compartment to expand it. At that point I might use the security strap depending on how much weight the bag holds:
Top has two clear pockets for GPS or Phone or whatever gadget you want visibility of. They're non-removable but the one in the front tilts:
Top with exterior pocket tilted, You can see one of the exterior side pockets (one on each side) I use them to store maps in:
Inside. It has one mesh pocket on the right side to store things you don't want to be searching for if the bag is full of stuff:
The Tourmaster from the side has a much lower profile and is non-expandable. It too has two exterior side pockets. Note that the magnets are in the side flaps extending down the tank. VERY secure!:
Interior is nicely appointed. Especially the inside of the lid as it has smaller pockets for pens. glasses and so forth:
The TM has a huge removable map pocket so you can sit in a café and review your route:
With map pocket removed. Utah sun is brutal on everything.:
I also have matching soft panniers for the TM. Same build quality although the rain covers have the inside lining peeling off (probably due to Utah summer heat). I run the straps under the seat and they have a number of adjustments and easily removable mounting points.
The Nelson Rigg doesn't have a dedicated pocket for its rain cover so you carry it inside the main compartment. Not a problem because it's cavernous. The TM has a dedicated pocket for it up front. Nice touch:
Also: the Tour Master has a built in carrying handle and the Nelson Rigg boasts ports for a hydration tube and exterior clip to hold the hose, or electrical cords.