They are 2 very different tool sets.
While you can do a lot of colour correction work in FCPX it is limited.
Resolve gives you tools that take you beyond simple colour correcting so that you can grade footage with masks, qualifiers, tracking, etc.
As an example. Lets say you have a blue sky and you want it to be more dusky in colour but you don't want to change the skin tones of the talent in the foreground. You can create a key based on the skin times and then track a mask that follows the talent. This would then allow you to so a separate correction on each. While this can be done in FCPX it is clumsy and the controls are limited.
There are many other things you can do by compositing multiple elements or layering footage and using bland modes.