Alistair, when you are considering the replacement of a printer, especially a large format printer, there are more things to consider than what you wrote here. The very first issue I would ponder is the longevity of HP's presence in this market segment. The model is by now very long in the tooth and they have said nothing about updating it - at least that I know of. The company is also going through a lot of "transition", to be polite about it, so the very first concern I would have is about future support in terms of their commitment to help, maintenance, consumables, replacement parts. I would want pretty solid assurances on all that before buying one.
Differences of colour gamut between various ink/paper combinations would show more in some images than in others, so this is not a constant, and depends AT LEAST as much on the paper as it does on the ink and the profiling. Since that printer was manufactured, other manufacturers have been producing models with expanded gamuts relative to their own previous editions, but how they compare with a Z3200 I have no idea. This is the kind of thing that would be best assessed if you could work with a dealer who is set-up to let you see for yourself using several different kinds of images printed in several different printers on the papers you prefer using.
Quite apart from colour gamut, there is accuracy of ink placement, which depends on the screening and dithering algorithms embedded in the printer firmware. Smoothness of tonal gradations and the accuracy of detail rendition would be affected by this. Whether you can actually see these differences in the kind of output you produce again is something you should test personally before buying.
Finally, how you intend to use the printer is really important these days. If you intend to print only between longish intervals, any of the Epson x900 printers may not be a preferred option, because they are intended to be used at very frequent intervals, and if they aren't, a number of people are reporting clogs that take time and ink to clear-up. However, if you don't need a roll holder and 17 inch width is good enough for you, the Epson 3880 is a very reliable, relatively trouble-free performer producing beautiful output. People who print infrequently and want wider format than 17 inches should consider Epson's non-x900 series as well as Canon offerings, by doing research on performance for specific models and looking at printed output using their preferred papers and test images.