Scratching the tank should not be an issue with strapons or magnetics. I've used both extensively on several bikes. My biggest mistake ever was to set a magnetic bag on the side of the road as i had to make an adjustment. Some gravel embedded itself in the soft vinyl belly. Then i put it back on the bike. Under some really hard braking it skootched forward, the gravel acting as marbles, and scratched the tank.
Moral. Always clean off the bottom of the bag, assuming its off the bike at some time.
As to the filler cap, a strapon will sit on top of the cap better than a magnetic. You lose some interior room as the floor of the bag is pushed up. Other than that its fine.
As to which one to use, Google it. I've used RKA, Eclipse, Tourmaster and a few others. No recent purchases though. And the lineups change every season it seems.