What type of blends are you smoking, as it makes a difference. As far as the sizes, you mention, the top is a large group 4 or a smaller group 5, the second one is a group 3. Those of us who understand the Dunhill sizes can use them to our advantage especially when buying pipes on either ebay or at a retailer. Once you learn that the Dunhill sizes are a guide, you too can take advantage of them.
I only buy group 4-5 sized pipes in certain shapes. I only smoke flakes so I buy my pipes based on how they will smoke my flakes. I am not buying group 6's and Oda's with a 1 inch chamber diameter, pipes that large are good for complex English blends with multiple components, but too big for flakes. I can normally smoke 1-2 hours with my group 4 and 5 sized pipes.
Oh yeah, this is America, the metric system does not count here, so never use it as it offends my sensibilities.