Just to put my two cents in as someone who has recently bought a 7900-- I get pretty obsessive about black and white printing and I really feel like the gloss differential is not an issue at all. HP and Epson take two completely different approaches to this obviously. I have not printed a single print yet that exhibits gloss differential at anything resembling a normal viewing angle. I just went through some prints holding them up to the light at a fairly extreme angle-- you would really have to go through a bunch of prints holding them up to the light like this to find the right print with extreme whites and blacks. Maybe with the HP's gloss layer, you wouldn't even be able to do this-- but I'm really talking about holding the print up to the light and looking right down the surface.
I don't have the breadth of experience that Wayne and some others here have across brands. I have seen quite a few prints from the Z3100 including some really outstanding black and white prints. I came down to choosing between the HP Z3200 and the Epson 7900. I agonized over image quality even though I knew they were both capable of genuinely outstanding output. Little things pushed me over the edge to Epson. Ease of use is definitely secondary to IQ, but if the IQ is outstanding, the little things count for something. e.g. the paper handling for both roll and sheet is extremely easy. The paper cutter can cut canvas and I believe is self-sharpening. Finally, I was already an Epson person so it was just easier for me to stick with a brand with which I was already familiar.
Is it perfect? No. MK/PK switching is quite easy, but I still think it's silly that they share the same head instead of adding a new one-- seems to need a few minutes of a cleaning cycle whenever I switch. Everything is solid, well-designed, and over-engineered, except . . . . what's up with that paper catcher? It seems like something that Magyver tossed together with whatever materials were on hand. But, I just have the printer wait to cut or hold the print after it's done printing. I cut/release the print manually so it's not a big deal.
So, maybe not perfect, but pretty darned excellent. Black and white output-- shadow details, tonal range, punchy blacks on gloss-- is truly outstanding.