In this thread (I hope this is the right link...my computer is messing up) I have a pic of the cognac color and the desert color on a Chiara and a Sienna.
I didn't end up keeping either bag, but you can at least see the differences.
I have heard problems with the Desert color either fading or suffering transfer stains- probably because it's a lighter color. Between the two, I would pick the cognac!
I have the Sienna in desert and even though I just used it a couple of times (10 the most) the leather gets a dirty looking patina and it stains easily. I would never buy this colour again as my hardly used bag looks like 10 years old.