SmearFX is more specialized in pixel art images, and has a quicker workflow than SpriteMancer. Making an smear animation in SmearFX can take (literally) two clicks, while in SpriteMancer there's more stuff going on and everything takes more work.

To give a silly example, If you want to do a quick sketch or paste a print screen image, you are probably going to open Paint instead of Photoshop because It's easier to work for certain things, despite both being capable of doing the same thing.