I would look at itinerary before worrying about the ship. La Paz and Loreto are smaller and less developed than Maz and PV. Your choice.
Now, for the ships, there is one important distinction if you are planning on having your kids share your cabin. A mini-suite is almost required for four in a cabin, and the mini-suite balconies are roughly twice as big on Diamond.
Most mini-suite balconies on the new ships are only 4 feet deep. The Diamond has 8-foot-deep (or is it 9?) balconies on minis.
Jim