Safety vests: Yellow vs. Orange?
The short answer is that, in practical terms, the difference isn't great. The longer answer requires some explanation of "visual conspicuity."

In optical engineering, conspicuity is the study of what makes things "conspicuous," and some researchers split the tasks into "detection" and "identification" (see, for example, the works of A. Toet et al, such as this one). Because the rods and cones in our eyes have differential sensitivity to light and color, some colors are more easily detected in low-light conditions, such as dusk or dawn (the rods are more sensitive than the cones but are essentially insensitive to color -- that's why all cats are grey in the dark). However there's still the "identification" task, and that's where the choice among high-visibility, naturally-rare, colors matters.

In general, in order to be conspicuous, you want both to be detected and to be identified as a cyclist. Experiments with the visually impaired (and color blind) show that contrast is the key. Contrast can be achieved by shape, motion (which is why emergency vehicles have blinking lights: to achieve "optical" motion), and color. What this means is that the color you wear should be differentiable from the background, so you want a color that is both detectable in a crowded visual field and also that is uncommon -- or rather, uncommon compared to other objects in the visual field but common to cyclists so that you can be identified as one (cf. here and here). The fluorescent colors of high-visibility yellow-green and high-visibility orange are pretty unnatural so the likelihood that it will be "lost" in the visual field is low but in addition, the fluorescent yellow-green seems to be commonly used only by cyclists and emergency workers (like firemen) -- at least, in the US. Conversely, hunters wear orange both because it's not just a good idea, it's THE LAW, but also because it's instantly identifiable by the conventions of hunting.

So the bottom line is that both are detectable though there may be some (slight) advantage to using the color that is most commonly associated in the public view with cyclists since it is both highly detectable and also identifiable. Should you be riding in an area where another color is (predominately?) associated with cyclists you might decide, of course, to use that instead; for example, during some long-distance randonnees (see, for example, the requirements for the quadrennial Paris-Brest-Paris ride), the organizers may require that the cyclists wear a belt or vest or other device of a particular shape or color so that drivers can instantly recognize and identify the carrier of such a device as a cyclist.