The 603 is awesome. Just think of a full size boat getting almost 30 mpg on the highway. My 87 300D is quicker than my 380SL. The 603 is smooth, quiet on the highway, and pulls through the entire rpm range like a train.
The only time you know its a diesel is at idle.

+ no valve adjustments.
+ better fuel econmy
+ more power
+ smoother, quieter
+ simpler oil changes due to better oil filter setup.

- extra maintanance on cooling system
- glow plug job a little more involved

Some maintanance easier on 603, some 617.
trap ox should have been upgraded to a new catylist further downstream.

Both capable of going 350,000 miles

__________________
1959 Gravely LI, 1963 Gravely L8, 1973 Gravely C12
1982 380SL
1978 450 SEL 6.9 euro restoration at 63% and climbing
1987 300 D
2005 CDI European Delivery
2006 CDI Handed down to daughter
2007 GL CDI. Wifes