My take on this is that you may get more accurate color rendition outdoors using films that are not "Extra Color" or the Kodak "S" series. (S=Saturated). I believe these films are Kodak's response to certain of the popular Fujichrome films, which tend to be more colorful (saturated) overall. I base this statement on my own experience.
About 5 years ago when Fuji Velvia was the up and coming buzz, I conducted two controlled evaluations with identical cameras/lenses. This work was done to satisfy my own curiosity, though some of it was used in a film comparision study the by magazine Railpace.
The shots were outdoor subjects, much of it scenery. I had Velvia in one camera, K64 in the other. Velvia's color saturation (and tight grain) brought many "Oohs" and "Ahhs" when I presented the results as slide shows. These shows were comprised mostly of experienced photographers. Many were K64 shooters, but the Velvia surely impressed most of them. Of course, we didn't think Velvia's colors were more accurate, we just thought they were more eye-catching.
Regarding some of the follow-up posts. I sure like US Airways current c/s, like on the great RJ-145 shot that was posted, but we really need more colorful subjects to evaluate the film. But really, it's up to you what level of color accuracy you want in your slides. I prefer a film that is more accurate and life-like than the new "pumped up" films.