The 603 engine is powerful, less maintenance, better fuel mileage, less noise. But it comes with a price. The parts for 603 engine are usually priced more than the 617's. For example, vacuum pump, $400 for 603, $230 for 300SD (617). Other items, such as, fan belt, fan clutch, oil pan, coolant tank, are generally higher for 603 engine.
603 has an aluminum head which is expensive and easy to crack or wrap, especially the early model. I think the 617 has a better timing chain than the 603.
Stay away from 350SD/SDL.