To me, why I would want the SFX coaster more-so than the Mack InvertPow Coaster; is that it would keep a balance in the park of having something with a high thrill factor next to Potter, while also being heavily themed.
Although, I did mention that if it's not the SFX and is the Mack InvertPow; then I am actually happy about it for a few reasons more than one. One in which, being that it would bring more potential unique showscenes for the attraction in many way's more than one.
Both I would be perfectly happy, but I'd hope to see the SFX a bit more than the InvertPow.