Hi Gerard, my name is Jason.
There are pro's and cons to each engine.
The D3 has the newest technology Volvo has to offer. It is lighter than the Kad32. It uses common rail technology for better fuel efficiency. And the boat will perform better with it. The cons are that because it is so technologicaly advanced, if you ever have a problem with it that requires troubleshooting the engine will most certainly have to go to a dealer. They are also not designed to be rebuilt. If the engine ever wears out, or has a catastrophic failure, that would be the end of the engine and you would have to buy a new one.
The KAD32 engine is much more rugged. It has removable cylinder liners, and if it ever wears out or has a failure, it can be rebuilt. It is a little more basic technology wise, so anybody with a diesel background can typically work on them. They are not the most fuel efficient engines (by todays standards). And they are also heavy. If you take 2 identical boats, the boat with the KAD32 is going to be both slower and use a bit more fuel.
As far as which would be the best choice. From a personal standpoint, if I was going to buy a boat and planed on keeping it 3 to 5 years, and then selling it for a bigger boat, I would choose the D3. But if I was buying a boat that I wanted to keep for a long time, longer than 5 years, personally I would chose the KAD32.