As I was redoing my desk, I wanted a new mouse. For light editing on the laptop, the touchpad is fine, but for more detailed editing then I reach for a mouse on the desktop. If its going to be a long detailed editing session then nothing beats a pen and tablet, so I also have my trust Wacom.
For a mouse, comfort, a few extra buttons one can configure for extra functionality in Lightroom and Photoshop, and a way of customising the scroll wheel. MAC OS still has not implemented a way of setting different scrolling preferences between touch pad and mouse, so it set the track pad how I want it in the OS settings and set the scroll wheel how I want it in the Logitech software.