Having recently been forced to use Illustrator for the first time, and having used Photoshop for a while, I can see why they are a lot more expensive than RISC OS/Linux 'equivalents'.
They just have so much more in them (certainly in respect to the day-to-day requirements, rather than the 'really cool to have, but when is it ever going to be used in real life' requirements). The only things that let them down are they keyboard shortcuts (although they are redefinable).
Although I've not used ArtWorks and PhotoDesk for a while (and never used Vantage), I feel that there is still some work to be done to make them to the same level as the more expensive rivals (although Martin Wuerthner's work is excellent).
The only reason why I'd prefer The GIMP over Photoshop is that The GIMP produces smaller JPEG files. Oh, and the office fileserver sucks when it comes to file permissions, so I'd prefer to use my FreeBSD box instead of 'doze.