Both the Canon and HP printers are 12 ink printers, the Epson is an 8 ink printer. With the Epson, you have to swap the blackest black cartridge when you switch from matte paper to glossy paper. This will cost you ~$75 in ink.
From an image quality standpoint, there isn't much difference between the three printers. Differences are very slight. RIP stands for Raster Image Processor. It is a piece of software that optimizes image output. Colorbyte's is very good. While not absolutely necessary, if you want the very best image quality possible, you'll most likely end up purchasing a RIP.
Here's a link to an article that evaluates the 24" version of the HP. The author has also evaluated the comparable Epson and Canon offerings and references them here: http://luminous-landscape.com/review...0-review.shtml
. Technologically, the printers are identical to their larger brethren except for the maximum print size.
If I were in the market today, I'd be purchasing either the Canon or HP. As the article mentions, the HP has some really nice features built into it.