I mainly use a 9 inch grinder for cutting on large areas as it cuts so much quicker. Having said that, Variable speed polishers aren’t as harsh to use when polishing small items with edges. Also the variable speed polishers are smaller in size which make them easier to do bull bar uprights and tight areas. I nearly always use the variable speed polishers for final finishing. However, I often use the 9 inch grinder for finishing tanks and wheels. Don’t stress if you only own either one or the other, as you can do every job with either. Any brand polisher will work fine.