I have both and find the RO125 more versatile as it is lighter and my go-to sander for most things. That said, if you are only going to use it for large, flat areas (like cabs and playfields), I could see where the 6" would cover more ground faster so would be worth it. However, I wouldn't want to use the larger sizer to sand cabinet frames, vertical surfaces or anything above my head so would stick to the 5" if you plan to use it for other things. You'll be happy with either one...