Big difference. In Afghanistan, the natives wanted the invaders gone. The Crimean citizens will fight just as hard as the Afghans did. If Ukrainian tanks roll in, it will only grow the widespread belief that the Ukrainian government is controlled by Svoboda who want to ethnically cleanse the Crimea. Vietnam shows what happens when the local population is not on your side.
Probably 10-20% of Afghans wanted the Soviets there. And they would have held out if Yeltsin hadn't cut off their oil. Even if you add all of Crimea's population together, you don't get to 10% of Ukraine's population.