For what it's worth-
I've owned a Z3100 since May 2007, after three years with an Epson 7600. I'm delighted with the print quality of the HP printer, the painless (and free!) switching of black inks, the superb black & white output, and the built-in i-one spectro. If I were starting over today, I'd make the same decision.
On the other hand...
The Epson 7600 is built like an anvil, and never breaks. In my rather humid location ink clogs have not been a problem, and the straight-through paper path of the Epsons permits printing on basically anything, including 500 gm/m2 board. They are also the gold standard, with a huge network of third-party support and generic profiles available for them. I must also add that after more than 3 years of use I never had to call Epson's tech support or service even once. If you can stand the obnoxious black-switch issue, and if the newer 7880 is as reliable, it's a great choice.
The Z3100 by comparison is a bit more finicky in its paper handling, it does have issues with roller marks on some of the newer "fiber-gloss" papers, and you may need to pull the heads to clean off the ink overspray after about 6 months of use. Then again, the heads are pretty cheap, and you can pull them yourself without tools to see what's going on. My experience with HP tech support and service has been excellent. Again, I would still go for the Z3100 if I were starting from scratch, as black & white makes up about half my work and this is a terrific printer for monochrome.