Your perspective is perfectly valid, and many people are happy using PS/2 mice on their Amiga. But may I add my personal view of things?
I was happy with the old tank mouse for many years. You have to clean it regulary, but it pays you with pixel-perfect control. The smaller mouse that came with my Amiga 3000 was quite decent too, even though it was a little too small for big hands. I eventually switched to a trackball for my Amiga, as desk space became important.
I am not sure what the A4000 shipped with, but I have tried many different mice on PCs and Macs and only very few combinations will allow you to draw a "circle" without dents or holes free hand in a pixel oriented program. And I never needed a middle mouse-button except in X11 or Oberon.
So, maybe you should embrace the old rodent and give it another try, before buying a cocolino. Try to remember that smooth movements are better for your hands, and you will notice that a rolling ball can pick up these very well.
This is due to how the mice communicate with the Amiga. In PC mice, the mouse has to decode the movement and re-encode it as a message to be sent over serial etc., whereas the Amiga reads the optical encoders directly, allowing it to choose when to move the cursor. I've always preferred the control real Amiga mice give, but I have to say that I miss my scroll wheel terribly when I don't have it...
