In Idaho and Utah, the Oregon race of Dark-eyed Junco seems to be the most common. Every now and then you find a Slate-colored Dark-eyed Junco in the west like this guy that showed up in my backyard. Notice how it has the dark flanks? The feathers are basically just black or white. During my couple of months spent in the east last fall, I observed that almost all the Juncos were Slate-colored, with only the occasional Oregon. No matter where you live, when you get the less common Junco in your backyard it's a nice treat!
Another tip I came across somewhere, that seems to hold true most of the time is that Slate-colored Juncos have a concave dark bib, while the Oregon has a more convex dark bib as emphasized with the red curves.
Oregon Dark-eyed Juncos have brown to peach flanks and more brown on the back and upper parts, so their feathers are all around more colorful than the Slate-colored. I'm not sure if it is true or just my perception, but it seems that Slate-colored Juncos are slightly larger and broader chested, and that's not just based on these photos.
The first Dark-eyed Junco I ever saw was a Gray-headed in the mountains near Payson, Arizona. I was also fortunate enough to see a Yellow-eyed Junco at Ramsey Canyon, AZ which is considered a different species altogether. There are a couple other Dark-eyed Junco subspecies, but I don't recall if I've ever identified one in the field. Just another reason to keep on birding!