Except - the original Amiga mouse is that two-button one, mechanical and all.
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.
