Rail pressure gauges are only relevant to third gens. Third generations also have the option for digital gauges.
Instead of rail pressure gauges, first gens can get quite a bit of use out of case pressure gauges. The first gen may not have a tach.
Both trucks are going to need boost pressure and pyrometers. Those are the most important. Fuel pressure is good for both years of trucks.
Transmission temperature is only important on automatics. You see a few guys with them on manuals, but that's rare and pretty irrevelant for most.
What else would be good? Drive pressure gauges are cool, especially so if you're playing with a second turbocharger. Intake temperature is good to have, especially if you are running some sort of fumigant like nitrous or water.
I've seen water temperature, cylinder head temperature, oil temperature, oil pressure, differential temperature... Lots of stuff that most wouldn't consider worth the price or space of a gauge.
Another option to consider would be a Cooling Mist "Smart" controller connected to a milspec laptop... Not too much more than "real" gauges, but you can control stuff with it.