12 valve: old school but tough as all heck. P7100 pump, mechanical lift pump, timing not continuously variable, all mechanical adjustments. The best for big cheap power. No computer chips here, just good ol' tinkering under the hood and pumps that never quit. Reliable and affordable, 1994 to 1998 saw the 2nd generation of 12 valves. 1st gen 12 valves ran from 1989 to 1993, and were also reliable as heck, but had a mechanical VE rotary pump.
24 valve is everything newer in one way or another. The 1998.5 to 2002 trucks were the VP44 Injector Pumped 2nd gen 24 valve trucks. The electronic LPs on these trucks were prone to failure, which if left unchecked would then take out the VP44 IP, and it became an expensive mess. Fuel pressure gauge is a must on these trucks. They have continuously variable timing, elecrtonically controlled, and were the first Cummins Dodges to accept aftermarket performance computer controllers or "boxes." Big power, big noise, and some headaches can be found with these rigs, but the engine is still a Cummins and never quits.
2003 to current Dodge Cummins trucks are still equipped with a 24 valve head. As the years progressed, things like galley cooled pistons came on scene. These 3rd generation trucks have the same electronic lift pumps overall (some on engine like 2003, and then in-tank mounted later on), but have a CP3 Common Rail injector system. The CP3 pump pressurizes a common fuel rail that serves all 6 injectors. The injectors themselves are electronically controlled, allowing for multiple injection events per combustion. These trucks are also capable of big power. Performance boxes on these trucks can address injection duration, rail pressure, and timing. Also, the CP3 is proving to be more reliable than the VP44, and these engines are much quieter than the 1998.5 to 2002 trucks.
The VE in the name of the Bosch pump stands for "Verteiler", which is German for distributor or divider. The first gen trucks had these.
Clicky here for VE pump info: Robert Bosch type VE Diesel injection pump
You're not going to get the huge power out of a VE pump compared to a P7100. Also realize that the VE trucks were the old first gen body style. To get the 2nd gen body style and the P7100 pump, look for 1994 to 1998 12 valves.
The crown jewel? Find a 1998 12 valve extended cab. They had the new interior, the extended cab rear doors, and still retained the 12 valve engine. Later 1998.5 trucks look the same but have a more problematic early 24 valve VP44.
So, early 1998 12 valve is what you should strive for if you can.
First gen VE trucks are affordable, and are tough as nails. Depends on your budget and HP/TQ goals.