My personal take is that the Quest is the direct replacement of the GPS V, and the indirect replacement of the SP III.
The SP 26xx series accomplishes much the same tasks as the Quest, but with a larger screen, larger memory capacity, and additional features such as a touchscreen and a remote.
European purchasers are very lucky, because we really do have a choice between a Quest with sufficient memory (243 megs built-in) and a SP 26xx. North American users can only purchase a Quest with about 115 megs of memory, which makes it pretty marginal for motorcycle touring.
PanEuropean