One originated in Mexico, while the other dates back to Imperial Russia. For little dogs with such different backgrounds, they look a lot alike. While the Chihuahua is eligible for American Kennel Club registration, as of 2014 that's not true of the Russian toy terrier. However, the breed is recorded in the AKC's Foundation Stock Service, which permits unrecognized breeds to continue developing.
Both breeds come in long and short-haired varieties. The Chihuahua breed standard permits any color, whether solid or with markings. Permitted colors for the Russian toy terrier include solid red, black and tan, brown and tan, blue and tan, sable, and brown sable. The latter shade is reddish with a brown overlay. The long-haired Russian toy terrier might not develop his full coat under the age of 3. For Chihuahuas, only the "apple" head type is permitted in the breed standard, although the "deer" head with the longer muzzle exists. The Russian toy terrier's head should appear smaller in comparison to the body, with a high but not overly wide skull.
The Chihuahua breed standard doesn't permit dogs over 6 pounds, with most Chis weighing between 4 and 6 pounds. Of course, some Chis are larger, but they can't compete in AKC dog shows. The breed standard for the Russian toy terrier allows a maximum weight of 3 kilograms, which translates into 6.6 pounds. There's no height standard for the Chi, while the Russian toy matures between 7.5 and 10.5 inches tall at the shoulders.
Chis generally bond tightly with one person. They're tough, fearless little dogs. While a Chi is too small to protect you against an intruder, he will certainly raise a racket. The Russian toy terrier tends to be more playful and active than the Chihuahua. The Russian Toy Terrier Club of America website states that there's a temperament difference between the smooth and long-coated varieties, with the smooth coats exhibiting more terrier-like behaviors.
If you want to add a dog to your household fairly soon, it's likely you won't have to look long and hard for a Chihuahua. The popularity of the breed means you can probably find a suitable dog from a reputable breeder or rescue group, without enduring extensive travel. A less common dog, such as the Russian toy terrier, might take longer to acquire and the odds are greater that you'll have to travel to find a good breeder.