Messy and Forceful are interpreted by the GM; the object of the Smash! is named by the player.
As a GM, if someone dealt only 1-2HP of Messy/Forceful, I'd say they slashed the orc's face open, and blood sprays everywhere, and the orc falls back a pace -- but I wouldn't dismember the orc. With Smash!, it's not up to me; if the player hits 12+ and wants the orc to lose an arm, it's gone.
Messy/Forceful also won't usually take away the particularly important magic item your opponent is wearing. Smash! is ideal for getting Magneto's helmet off.