I can't answer the question about which string is more powerful.
However, I do know that Tonic is essentially VS with slight gauge and color imperfections, at 10$ cheaper. I'm pretty sure all else is equal in terms of manufacturing process, materials, etc. Manufacturing gut is a long, complex process and sometimes will have imperfections--at your wallet's benefit
It's not going to be easy to get an answer because not many people have tried the FXP Power, but what I can tell you is that the VS is very powerful to the point of being uncontrolable. Other multis that are supposed to be powerful like NXT are lifeless and dead compared to the VS so I doubt that the FXP Power is much different.