Even their homeworlds! One an arid desert, the other a frozen wasteland. One has no moon, the other IS a moon.
I favor the idea that Vulcan is also a moon, in orbit around a gas giant. This would nicely explain the view of the Vulcan sky in TMP. And it would explain how Spock (in ST: Eleven) could be close enough to witness the destruction of Vulcan, he was on another inhabitable moon in orbit of the same gas giant.