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.
It's not canon, but the Star Trek: Star Charts book simply has the moonless Vulcan locked in a close pirouette with a large sister planet (T'Khut) that does
have a moon, with both being clearly visible from Vulcan on a clear afternoon.
It puts forth the idea that not all star systems are as neatly arranged as Sol, with planets and moons sometimes circling themselves (around a common center) as they orbit their suns.