I'll try to summarize, though when writing about printers I tend to go off the deep end ...
There are three dominant pigment ink sets on the market now: Epson's UltraChrome K3, Canon's Lucia, and HP's Vivera. K3 is an 8-ink system whereas Lucia and Vivera have 12-ink sets (for Vivera, the 12-ink system is in the Z3xxx series, not the Z2xxx series).
There are strengths and weaknesses among all of the ink sets themselves, not just the printers.
It is natural to assume that the 12-ink systems will have a larger gamut and/or a higher d-max than the 8-ink systems. Practical measurements show this is not the case, at least not yet. The K3 gamut, for instance, is essentially the same size as the Lucia and Vivera gamuts. The question is the
shape of the gamut. Certain ink sets are better in certain colors. Example: Epson is unrivalled in the mid to dark reds. Even the 12-ink Lucia and Vivera systems don't compete in the reds. But Epson falls behind both Lucia and Vivera in the mid-to-light blues, where the latter systems are particularly vivid. Tradeoffs in the ink formulation and design.
Another difference among these ink sets that most users are unaware of is that Lucia and Vivera are designed for thermal print heads (since the Canon and HP printers use thermal print head designs) whereas K3 is designed for a piezoelectric print head. Why does this matter? Because it affects the ink formulation, which in turn affects the list of compatible papers (i.e., which papers you can use the inks with). Epson has been in the market for a long time and consequently most of the third party manufacturers make sure that their stuff works with the Epson inks. Some of these papers don't work so well with Lucia and Vivera inks because of the differences in the ink formulations. For instance, Hahnemuehle Photo Rag doesn't work properly in color mode with the HP Vivera inks. This is why HP commissioned Hahnemuehle to produce a special version of Photo Rag (called HP Smooth Fine Art) for the Viveras.
There are complications with creating ink systems with more inks in the set: makes it much more complicated to drive properly, makes it harder to profile properly, and of course it makes the printer itself bigger.
For glossy, luster, semi-gloss (i.e., reflective) prints, the Z series printers have a noticeable edge because of the gloss enhancer. Really helps with minimizing gloss differential.
For B&W work, the Z3xxx series also have the edge because when you print an image (even if it's in color mode) and the R, G, and B values are the same as each other, then the printer only uses the 4 black inks and you effectively get a quadtone B&W printing system which has excellent tonality and is dead neutral. Plus, the black-only system ensures even higher longevity.
The HP Vivera's bare-bulb longevity is outstanding, definitely a step beyond what Lucia and K3 offer. See
http://www.wilhelm-research.com/ for details.
In terms of features, Epson has fallen behind in terms of maintenance and usability, things like head cloggings (happens to some people, doesn't happen to others, but it DOES and CAN happen) and the swapping of Photo Black and Matte Black inks. HP and Canon have stepped forward here and I expect Epson to do the same in their next generation. The funny thing is that Epson _did_ have both MK and PK inks available simultaneously with their older 4000 printer, but then took a big step backwards with the new K3 printers in this regard. Oops.
I agree that auto-calibration and auto-profiling will become standard in the future high-end printers (24" and up). HP has taken a great first step here, though it would be great if their solution was more flexible. For instance, there's very little control and flexibility offered with the built-in profiling solution. (One has to shell out $600+ for the Advanced Profiling Solution to get more options, and even then there are quite a few limitations.) Some competition in this area from the other vendors will probably fix that.
I don't think the built-in spectro will trickle down to the 17" printers, but it'd be cool if I was proven wrong.
I do think that the HP Z3100 represents the state-of-the-art in current pigment printers from a features, usability, and technology point of view. (Quality-wise, with the exception of the gloss enhancer, it's about a wash compared to the best prints from the Canon or Epson printers.) Definitely the best choice among the 24" printers. Be advised, though, that HP is still experimenting with the beast and haven't locked down the firmware yet.
As for myself, I can describe the thought process I went through when I invested in my Epson 3800 earlier this year. I wanted to experiment with and print on many different media types (glossy, luster, f-type, matte, etc.). I wanted to use larger carts so I wouldn't have to switch so often (this ruled out the 13" printers). I wanted to make both color and B&W prints (all the 17" and up printers do great here). I didn't want to manually switch between PK and MK inks (this ruled out the Epson 4800). I didn't want to deal with the learning curve and print head warranty issues of the Canon iPF5000. I knew I didn't need roll paper support (an important consideration -- the 3800 doesn't support roll). And I wanted to keep my wife happy (this ruled out the 24" and 44" printers).
Seriously, I never print that big anyways.
Eric