Hi Saltcod,
Having owned and lived with a CC53 with DD671s for 12 years or so, I would look seriously at the CC41 you are considering with the 6V53s. Take a look at the maintenance records, have an engine survey done if you move forward, and don't discard a fine boat with fine power plants because of urban legend regarding Detroits.
I have no experience with Perkins engines, so can't contribute 1st hand information on them. However, bear in mind that maintenance by previous owners is VERY important, irregardless of engine type. Availability of parts is also important, as the routine maintenance items on all marine engines can be spendy, especially big-ticket items such as exhaust manifolds, risers, exhaust elbows, heat exchangers, etc. It's fairly straight forward to check the WWW for representative parts costs for each engine, and I think you'll find the parts for the DDs are relatively inexpensive and available, literally world wide. Someone posted that fuel injectors for DDs are found hanging on the pegs in the Ketchikan Walmart. True story. Waterpump impellers, complete water pumps, exhaust manifolds, complete in-frame rebuild kits, etc etc for most DDs are all over the WWW, at NAPAs everywhere, and Pilot gas stations in the middle of the stinking desert(!).
Are the DDs noisy? Not appreciably more so than any other equivalent engine in the same engine compartment, in the same boat, with the same muffling and engine room noise insulation. Are they less "efficient" than other diesel engines? Perhaps, if you get down into the weeds of brake specific fuel consumption as it relates to horsepower. In the real world, fuel consumption is a small fraction (<10%) of the cost of ownership of boats such as these, in typical pleasure service. Do they leak oil? Not appreciably more than any other properly maintained marine engine. The 671s have multiple blanked-off inspection ports and assembly bosses, as they were used in so many different configurations. Less so for the 6V53s. And it is possible to inspect during survey through these self-same ports such things as the condition of the piston walls, the pistons themselves, and the condition of the piston rings. Again, have a complete engine survey done if you go forward with the purchase, and you'll be money ahead.
As you can probably tell by this overly-rambling missive, I'm a fan of 2-stroke DDs. I wouldn't hesitate to own a boat with these powerplants, all things considered related to condition at purchase. Hope some of this helps.
Regards,
Pete