Mainstream home and semi-professional printers are designed for general-purpose printing, so there's some compromise between the need to print adequately on plain paper and produce ‘good enough' photo prints. If your main use for such a printer is to print photos, though, you should go for the ranges which have the same design aims.
Canon's PIXMA printers and all-in-ones straddle both markets, with some built around four-colour output and others offering six or seven colours, where photo printing is the main aim.
The importance of the extra colours is to help with lighter tones. Rather than relying on the whiteness of the photo paper itself to provide lightness and using fewer ink drops to create the hues, six- and seven-colour printers offer lighter versions of cyan and magenta inks, to provide a greater depth of colour even in the lighter registers.
Epson also has both Stylus and Stylus Photo ranges where, as the names suggest, the Stylus Photo machines are designed to cater for photo printing. HP's Photosmart printers are not quite so clearly designated, as there are Photosmart machines aimed at the general home printing market. The trick when selecting is to look for the number of inks. Although you may not believe that six and seven-colour printers give significantly better colour reproduction, machines built for these inks are normally designed for photo printing.
There was a time when all-in-one machines were always an engine generation behind single function printers. The print engine was designed initially for a standalone printer and then moved on into models which included scanners as well. Most manufacturer's are now using the same engines in both their single-function and all-in-one devices at the same time, largely because they're selling more multi-function devices than single-function ones.