I can't speak for WC3, but WC2 and SC are (were) great games. I can class myself in Guru level for both, happily (Warcraft Brag: Garden of War, 7 enemies. Starcraft Brag (before expansion): Completely fair custom scenario, 7 enemies, 0 kills, 0 razings), so I know what I'm talking about.
Although both games are worth your money, I would still definitely give the edge to Starcraft, since the fact that Warcraft's Orcs and Humans were almost literally
identical cut down a hell of a lot on the replay factor. When the only differences are that Paladins have better spells than Ogre-Magi, and the Humans don't have Death Coil, it kinda sucks having 2 sides that are so damn equal. The game also had a few other flaws, such as the inability to repair Ships/Catapults/Ballistae, and the fact that the Griffons/Dragons were essentially worthless.
When you play Warcraft, you learn the Human tech tree, and then discover that the Orc tech tree is just the same. When I got Starcraft, though, I experienced what can only be called "culture shock:" Learning Terran was familiar enough (especially to a Warcraft veteran), but I was simply
stunned when I got to the Zerg campaign. ("You want me to send my
Farm to go scout out the enemy? And then, I'm supposed to tell my guys to climb
into my Farm, and it will carry them across the sea? What the HELL???")
In short, I have nothing but praise for Starcraft's threefold complexity. Each race is completely different, yet very well balanced, and the manifold layering of strategies means that you can go for
months without learning all the tricks. I thought I'd seen it all when I found out that EMP Shockwave depletes Protoss shields....but then I figured out how to drop a nuke without leaving the red dot.
