Can't go wrong with a DD but as said above they will slobber a bit, especially if you use cheap oil or don't change it enough. Early DD series 60 has poor cam shaft "diamond" seal problems. Caused a mess up high on the left side of the engine. Best fuel mileage all things being equal and reasonably cheap to inframe (highly recommend you do the injectors at this point). Can't say about the big or small Mercedes, never had them. The new DD15 looks like a winner, got to see one and they are sooooo quiet.
Cats (not the 3126), while more expensive to buy, and more expensive to fix are VERY re buildable and if you shell out for the extra warranties Cat really is the best at standing behind them. The bridge engines of 2002 to 2005 (before ACERT) are awful on fuel. 3126's, well, I've had 5, good performers, but not cheap to fix. The HUEI fuel injection system is $$ to fix and if you have any oil leaks in it, they get big quick due to the pressures. Buy the warranty and you'll get better luck if you get them worked on at a "CAT" distributor.
Cummins, always seemed like it was a big fight for warranty. Likely had more to do with the distributor for Ontario who had to authorize the work. ISB was a good 400,000 mile throw away engine and the ISC was ok, both did not like being overheated as the heads would warp. The old N14, bulletproof and cheap to fix. The ISX, good performer with a great Jake brake. M-11's no problems except they leaked oil and changing a waterpump on one is my idea of hell.