Stick with what you have. The latest Epson generation is simply not as good as the HP's for black & white for several reasons. The HP "Z" printers let you use all four black/grey inks at the same time on some art papers, while Epson makes you pick gloss or matte black. The Vivera black/grey inks are absolutely dead-neutral on their own, while Epson's ABW mode has to mix in some colored ink to counteract their black/grey inks' warm color cast. The attainable D-max on gloss/baryta is similar between the two brands, but the Z printers yield a noticeably darker D-max on matte cotton rag papers. Finally, I think HP's black & white mode is simply more usable than Epson's ABW. It permits you to tone highlights and shadows independent of each other, and at least for me the results were more predictable and consistent.
There may be reasons to prefer the newest Epsons over HP's Z printers, but black & white output is not one of them.