I'll try to make this somewhat brief (hard for me to do, I yap too much). If anyone would like more detail let me know.
The real question being asked here is a Canon ipf6100 vs. an HPz3100. Canon sent me an ipf6100 about 10 days ago, and have enjoyed seeing what the printer can do. I spent about 2 weeks with a 44" z3100 a couple of months ago, and currently own 2 Epson 3800's, a 4800, and a 9800. I ordered the Epson 11880 immediately upon reading about it, and after examining prints made on the 11880, the 9880 and 9800 as well as Canon printers at photoshop world, I am excited about this new printer. I also still have an ipf5000 Canon sent to me shortly before they were introduced and tested it for a few weeks back then.
So I'm sort of an Epson guy, but I've had a chance to play in depth with all of them. From that perspective I will say I believe the 6100 is an outstanding printer, and I'm getting gorgeous prints ... every bit as good as my 3800. Not better mind you, but side by side it is incredibly difficult to find any difference, especially one significant enough to base a purchase decision on output quality alone. The new black inks do make a difference, and black and white prints I did on Museum Etching look really nice on both printers. I see less grain and smoother transition detail than I remember with the ipf5000.
My personal feeling is that I like the output from the Epson and the Canon better than the z3100. The HP is a great printer, but I found it more challenging to maintain delicate transition details, and struggled with reds, especially rich reds. The gloss optimizer is nice if you do a lot of photo papers, but gloss differential and bronzing really isn't much of an issue for any of these printers now.
I mentioned a few days ago (I believe in this thread) about challenges using the front loading manual feed for thicker material on the ipf6100. I finally did figure it out and now I'll admit it isn't that bad, so my only criticism of the Canon ipf6100 is again in documentation (it shouldn't have been that hard to figure out). The user interface for printing is still a struggle (let's be honest, printing interfaces should be better for all printers, but I think Canon is the worst of the 3). That being said, it is better than the ipf5000, and once you figure it out isn't an issue.
One final opinion when comparing the z3100 to the 6100 (or the Epsons for that matter) is in regards to the onboard eyeone and self profiling. While I commend HP for the idea, and I know everyone is excited about it, I personally think profiling is going in a different direction. The self-calibration of the new Canon printers seems insignificant, but in reality it is a big deal. It puts them at a level that Epson seems to have achieved, especially with the 3800, that being the printers are very consistent. It isn't that the printer is consistent with itself, it is consistent with other printers. Paper makers will be able to provide canned profiles that are equal to or better than you can make for yourself. I believe the Canon is consistent enough that most of the good media out there will have canned profiles very quickly. So it just doesn't seem to be worth the extra money to buy one in the printer. 5 years ago this would have been remarkable, but seems unnecessary now.
Please understand this printer may have issues that no one knows about ... it's too new. So my comments are regarding output quality, not build quality. It could very well be the roll feed system of the printer is flakey and a month or two from now the net will be full of people upset that they bought the printer based on what someone said. I will say the printer seems well built, and I had very little trouble loading paper with very few skewing errors, even with the front manual slot (once I finally figured how to use it with my preferred paper choices.) As far as build quality it isn't new, it is pretty much identical to the 6000, so instinct tells me it's going to be fine. But no one knows right now, so if that worries you, I guess you can wait or look at other makers.
Finally I do have a couple of broad recommendations. If you don't need roll feed, don't switch between photo blacks too often and only need a 17" printer, the Epson 3800 is a strong contender. Beautiful quality, and I have 0 clogs in over 2 months between 2 printers. If you need roll feed and need to switch blacks more than occasionally, the Canon would be my choice. I've heard the new 80 series reduces the amount of ink it takes to swap, but it will still take more than the Canon (which doesn't require any). If you are the rare user that prints on heavy stock ... especially heavy roll stock, the straight through paper path design of the 7880 is something you should look at closely ... the Canon does present some challenges there.
sorry, guess that wasn't too brief, but I hope it may be somewhat helpful.
