We had a '92 for about 5 years and now have two '96s. The difference in power is -very- notable.
The '92 had difficulties holding 75 on the interstate for anything resembling a hill. It was happiest at 60-65 mph. The '96 with stock tires (31") has notably more go than then '96 on 285s (33"), but that is expected. Both of the '96s go 80mph without much issue.
There are other differences. Since the '91 and '92 versions were not a long release in the US, finding parts for them can be more difficult. changing the exhaust out (manifolds, Y tube, cats, muffler) was a multi thousand dollar affair in the end, spread out over a few years. It isn't much cheaper on a '96 though.
The '96, despite the larger displacement, has a more organized under hood look and feel. IMHO everything is easier to get to. It also lends itself easier to dual battery mods.
Some advantages to the '91 & '92... The front console cup holder is usable for 16 oz drink glasses popular at fast food outlets. (In a '96 they only fit cans.) The dash clock has an alarm and countdown timer. It is dumbed down on the later models. The manual cloth seats in the earlier models are a better fit for my big butt and offer more leg room.
The '91 & '92 have stiffer springs and sit about 1" higher than the '96s do stock. Also, since they don't typically get the steps, they tend to have a more aggressive stock appearance.
The '92 CEL codes can be read using a paper clip if you know how to count.
The '92 rear axle is a partial floater Vs. the full floater in later models.
The '92's air conditioning is based on the 'old' refrigerant. It blows colder air quicker than the '96 will, but recharging the '92 is expensive.
It all depends on how you want to use the vehicle and what your budget is.
They're both serious trucks and a whole lot of fun. I bought the '92 for my wife to drive, decided I liked it, she wouldn't let me mod it so I bought her a Grey '96. She refused the Grey '96, preferring to keep her '92. I started modifying the Grey '96 A couple years later we decided she needed something newer. She had given the '92 several cosmetic dings. I gave her a choice between a new vehicle or another 80. I bought her a ruby red '96 and convinced her to let me sell off the '92.
We both love driving these. The maintenance can be a bit intensive and parts aren't overly cheap. They are, after all, $50,000 trucks and expect to be maintained as such. We plan to never sell the ones we have now.