Insanely difficult choice for me between the Saturn and DC, but the presence of Shenmue I and II on the Dreamcast makes it more essential. Only Saturn game to really come close for me is Dragon Force (and maybe Panzer Dragoon Saga, if it weren't so damn expensive). Furthermore, its potential for homebrew games and emulators really extends the Dreamcast's longevity, even in light of it not having as large a library as say, the Genesis/Mega Drive.
The only downside of the Dreamcast is that it isn't as well built as its predecessors. I've had a Saturn since 1996 (and one that I've admittedly been very rough with) which works flawlessly, whereas my first Dreamcast broke down after three years of careful use.