I have 2 completely rebuilt heads for a S350D.
numbers are 603016 17 01
and 603016 18 01
anyone know the difference and which would be better to use?

18 - 17 = 1. That's the difference

The #18 head casting is technically an improvement on the #17 casting, original on the 603.971. What the improvements are are beyond me. It could be a performance improvement such as better geometry or metallurgy, or it could be a functional improvement such as to support accessories in other applications. I've heard of an improvement in the #22 casting that you don't need much training to see. Otherwise, I don't think an MB tech can spot the difference between the heads you have other than the casting number. All else being equal, I'd use the #18 head.

Sixto
87 300D

As Sixto said, later is better. I believe the last mod was the increase the amt of metal between the chain cavity and cyl#1. That was a place the head gaskets failed.
DDH

__________________
'95 E320 Wagon my favorite road car. '99 E300D wolf in sheeps body, '87 300D Sportline suspension, '79 300TD w/ 617.952 engine at 367,750 and counting!

Thanks guys.
I will take another close look and measurement of the heads to see if there is any obvious difference.
I agree, 18 is theoretically/logically should be a better choice.