I like to give my buddy Terry S. a hard time about his "frog green" Z-Rex, so I hope he doesn't read this and find out that I actually like everything about this bike other than the color.
Back when the Bandit 1200S and ZRX1100 (and later 1200) were both in production, reviews of one always mentioned the other. And when you see one on the road, you'll often see it accompanied by the other.
These two old-fashioned naked standards handle about the same and have about the same ergonomics. The Bandit might have a little more get-go when the light turns green, but that's not as important as all-day rideability and comfort, and the ZRX seems to have it (although I've not ridden it more than an hour at a stretch).
In the end, appearance may be the biggest difference between the two bikes. While the ZRX always came in some variation of bright green, there were also royal blue, tarnished silver and tomato red models. The small, boxy headlight and fairing looks a bit like an afterthought, but it has an advantage of being fork-mounted, so the headlight always aims into a turn.
The Bandit's slightly larger fairing may or may not appeal to you (I've always thought it looked a little dowdy), but the color choices (midnight blue, dark teal green and candy apple red) were a little more my taste.
All things considered, had I not bought the Bandit, I would have been very happy to have a silver ZRX.