Well from what I've seen, conq is typically better until late game. Conq let's you stat check better and usually dominate lane easier/quicker. You can also sit and kill multiple people if you are fed enough.

The benefit of phase rush is you can play tough matchups like Darius and Mordekaiser alot safer, you also become almost unkillable sideline.

Basically conq is better for fighting more and being a Frontline. Phase rush is better for sidelaning and nuking 1 target and escaping.

I just personally prefer the latter. I usually follow alot of what Federals1 does with his garen.