I have the PO 39.5 and the Diver 300M. I really like both of them. If I had to pick one, it would be the PO. Mostly because I have smaller wrists, and the 42 mm is the max I can pull off. Side by side they look much different in size. If the PO was 1 mm bigger case, and 0.5 - 1 mm thinner, it would be the perfect watch for me. The PO is technically thicker than the diver 300, but it doesn't seem so on my wrist. Bottom line, is you have to try the PO on, and you might need to own both watches.