I’m on Team Both as well—trackpad to the left of my keyboard and mouse to my right. I can’t use the Magic Mouse though, the thing is an ergonomic nightmare.
For me, the usage is broadly split into navigation vs. interaction: I navigate pages, views, etc. with the trackpad and perform click operations with the mouse. I much prefer the trackpad for things like scrolling around because it’s a less mechanically complex motion and thus causes me less strain over time.