Hey,
As far as general care goes, I haven't read anything about one species of Collared being more difficult than any other. collaris gets quite large and can be quite beautiful, as displayed by eve's Kong. I think she said he's 16" total length. yowzaa! collaris are readily available, and often affordable.
I kept bicinctores, which don't get as large as their more Eastern counterparts, but again, they aren't always as colorful.
Also readily available depending on time of year.
vestigium and dickersonae are just breathtaking, but can be quite pricy. They are also somewhat harder to breed. (am I right Will or Brock?)
there are other species as well, but I don't know much about them to give you any idea as to which you should go with.
In short, you need a beefy lizard companion in a Collared Lizard home. don't keep them with smaller lizards of any kind. Your desert habitat will turn into a buffet. Another thing to note, especially if you have a Horned Lizard, is those little fellas stress out quite easily. My friends described an instance where they put a yearling Horned Lizard in a 100 gallon tank with a hatchling Collared Lizard. The HL was roughly twice the girth of the collared, and about as long. The Collared lizard went on the offensive, trying to size up this spiny oddity. The Horned Lizard flattened itself into a disk, and used its now broad, flat back as a shield to prevent further advances from the collared. If they didn't remove the HL, it would've succumbed to stress within weeks.
You can house other collareds with a Collared lizard. Chuckwallas will do well with them, as well as large Desert Iguanas. Just be sensitive to each lizard, and note its reaction to its cage-mates. Thats the advice I can offer you.
hope I helped you. I'd sure like to see the habitat when you finish it, so post a pic (preferably with inhabitants, lol)
regards,
-Phil