I have a HP B9180 and an Epson 3880. I know these are not A1 printers, but the inks are the same or at least similar to what is in the bigger printers.
To my eyes, the HP does much better on matte paper than the Epson, especially for B&W. The blackest black you can get on matte paper is noticeably darker with the HP. With some images, I find this can really make a difference. The HP also seems to work well with any matte paper I ever tried, I find the Epson more picky about which paper I use, at least if you want really good blacks. I like it with the Epson Hot press papers but find it kind of bland with other papers I've tried.
With gloss paper, the situation reverses. The HP ink just isn't compatible with many of the fine art gloss papers available. The ink pools up and doesn't spread out on the paper correctly. These are really tiny "pools" of ink, but makes the prints look grainy. The HP ink does work well with the Harmon Gloss paper and maybe other papers I never tried, but I tried a bunch of gloss baryta papers and none of them worked without the pooling problem other than the Harmon. The B9180 doesn't have the gloss optimizer of the larger printers so I don't know if that makes a difference. The Epson just seems to work with pretty much every gloss paper I tried. I like what I get on Canson and Ilford papers better than anything I go with gloss paper on the HP. I do think the HP did a bit better on the Harmon gloss than the Epson does on the same paper.