Before getting down to brass tacks, we need to talk about the inks that are the cornerstone of these printers. There are two technologies: solid inks and liquid, water-based inks. Solid (pigment) inks, as found in the Durabrite cartridges used by the Epson Stylus C66 Photo Edition, offer greater durability. According to Epson, the life expectancy of photos printed with Durabrite ink is 80 to 100 years due in particular to better resistance to light (we promise to get back to you on that). They're also much less vulnerable to humidity damage. In fact, we noticed that Stylus C66 Photo Edition prints can be held in your hand just after printing without causing the slightest smear. The higher durability of solid inks, however, is achieved at the expense of color quality; the output lacks sharpness and reminds us of film prints on matte paper.
The color inks used by most other models, on the other hand, deliver brighter, more brilliant colors, making them ideal for glossy prints. But they're less resistant to light, with a life expectancy estimated at 20 or 30 years. They are also vulnerable to humidity; it's better to wait for them to dry before handling them. (Canon even incorporates a system of latency time between two prints to allow one photo to dry before printing the next one.)
For its part, Lexmark has chosen to use a mixture of the two technologies. The traditional color cartridge (cyan, magenta, and yellow) uses colored inks, while the photo cartridge (black, light cyan, and light magenta) uses pigment inks. The manufacturer says that this allows the bright colors of the former to be achieved along with the durability of the latter. The mixture doesn't appear to have fully proven itself, though - the output of the Z816 in photo mode clearly lacks vivacity in the bright colors.
The quality of the inks is important, but so is the number of colors, and there are major disparities in this area as well. The Stylus C66 Photo Edition and the Canon Pixma iP2000 use only the four basic colors (black, cyan, magenta, and yellow). The Lexmark Z816, Photosmart 7450, and Photosmart 8150 add a special photo cartridge containing black, light cyan, and light magenta. This increases contrast, particularly in light-colored areas, which are often hard to reproduce correctly. However, the basic black cartridge has to be replaced with the photo cartridge when you want to use the quality output mode, which is a limitation.
The Photosmart 8150 also adds an optional cartridge dedicated to black-and-white and grayscale prints, and its results are very convincing. The Pixma iP4000 and Pixma iP5000 models operate with five cartridges; the four basic ones are backed up by a black cartridge dedicated to improving color shades. Finally, the R300 is the only printer that directly uses six different cartridges simultaneously, including light cyan and light magenta.