Do you have a buyer broker helping you? I am shopping actively for a trawler (or SD hull what some call "fast trawler") and my broker told me that there is one local shop who maintains Volvos and they are very expensive and parts can take a long time to get. Thus, because I will be living aboard, I have ruled out any boat that is powered by Volvos. I have also ruled out anything powered by Cummins VT903's, as Cummins no longer supports those engines (what I heard elsewhere).
So basically the powerplants are one of my "top 3" criteria for boat shopping. My other two are layout, and quality/consistency of maintenance (are there records, etc.)
If that Californian has cat 3208's, that would be a big plus in its favor, for me, versus the Volvos.
As far as Defevers go, I have heard nothing bad about them, as long as they have been well maintained. Some depends on your intended usage. Where will you be using the boat? If you will be in protected waters almost always, that may equalize the two choices a bit more. Also, where the Defever was built (by which yard?) may be something helpful to tell us (MMC, Bluewater?). I am unaware of which yard did a better job, but others here might know.
You will have to assess how well the layouts of each boat meet your needs. Overall, it would be helpful for you to provide more info, or links to the listings of each boat,