I'm very attached to my Cooler Master MM720. Out of all the mice I've used with good performance for gaming, it's the only one that doesn't aggravate my wrist pain. Typically shaped mice, especially narrow ones like the G403, force you to clamp your hand shut vertically with your wrist rotated in order to hold it properly, especially if you use a fingertip grip. That's a very unnatural position for your hand to be in and is guaranteed to cause problems eventually. The wider shape and dedicated ring finger rest on the MM720 feels much more natural to me.
I use a vertical ergonomic mouse at my work computer, but using them for fps aiming never felt right to me and most of them have bad sensors and input delay.
All of tragic's advice is good, too. Especially wearing a specialized brace when sleeping, that made a huge difference for me.