I have an older Late 2009 27" iMac that I'm getting ready to sell because we are purchasing a new one. I want to format it and load back on the OS. What OS should I use on it since its been update numerous times since I bought it. I have macOS Sierra installed on it now with 16gb of memory but I don't feel it runs that great or it may just be that it needs to be freshly installed. What do you think or what would you do?