[jclark003]

They are both very good engines if maintained properly and run within the design envelope. I believe all C-12's have the upgraded aftercooler design, released during the 3196 production years. Cat will tell you to replace them every 6 years in salt water applications but many owners have them instead cleaned, inspected and pressure tested. Cummins M11 also a good engine, a little less forgiving to over-propping and hard running due to the exhaust manifold issues. They also had some relatively minor issues with things like the front seal and a gear cooler redesign.

I wouldn't be afraid of either one of these engines provided you get a good engine surveyor and have full visibility to maintenance records. If you want more input go to boatdiesel.com, pay the $25 to join and have at it. Tons of information available in the forum archives from very knowledgeable owners and mechanics.