Physically, they're all much the same (1987-2007.) Technically, there are differences - and I'm assuming that's what you're referring to.
If you go OBD-I (1991-1996) or OBD-II (1997-2007,) you'll be wanting some sort of "code reader," and you may have issues with getting a MIL wired up into the dash panel. OBD-I will probably be easier for you, since you'll be able to fit up an OBD-I gage cluster (1991-1995/6, before the ChryCo redesign.) OBD-II drives all of the gages off of the CAN bus, which is already going to be a huge wiring job (moreso fitting the gage cluster, I fear...)
If you're going to do a swap of that magnitude, I'd suggest getting a compleat donor vehicle (rollover?) - not only to make sure you get everything, but so you can see where it all was before you get it out and move it over!
Still, I've found RENIX easier to work on than OBD-I/II - probably because I came up on points and condensers, and I'm used to doing my own troubleshooting. There's a good deal of RENIX technical know-how on this board, it's not a bad system, and most of the engine management sensors are General Motors parts - which makes them easier to find.
All else being equal, I'm probably going to tell you that it's easier to just learn the RENIX system (it's a fairly quick learning curve, once you get started. I can probably provide you with some tech material if you'd like,) than to undertake swapping to OBD-I/II. However, if your heart is set upon "upgrading," might I ask that you keep all the RENIX control electronics, include a note with detailed application information, and contact me for my address later? I'm working on reverse-engineering the RENIX system for a number of other projects, and donor electronics make me fear blowing something important rather less...
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