Here's a picture that shows the dark spot in the center of the gorget that means the bird is an adult female; it also shows the tail feathers. There are two differentiating features between Allen's and Rufous, both involving the tail feathers.

First, R2, the second rectrix or tail feather out from the center, is mostly symmetrically spear-shaped (lanceolate) in Allen's, while in Rufous it is asymmetrical, with a notch in the tip. See the illustration at http://www.pbase.com/tgrey/image/95431365 .

Second, the two outermost tail feathers, R4 and R5, are very slender in Allen's, relatively wider in Rufous. See illustration at http://www.pbase.com/tgrey/image/95431056 .

Neither of these criteria is really decisive here. R2 looks lanceleolate to me, but the angle isn't straight on, and you can't really tell. R5 looks slender, but again the angle is such that you can't really say with confidence that it's too slender to be a Rufous.

So, as with many adult female Selasphorus hummingbirds, we have to leave this one unspecified; it could be either Rufous or Allen's.