OnyX is free. Cocktail will lighten your wallet just a bit. They are both GUI frontends to BSD tasks. The two utilities are not antagonistic in any way. Use either or both. I don't see the point of paying for Cocktail, but that is your choice.
I use Onyx on both of my computers from time-to-time. I really don't see much of an affect, but I know Onyx is cleaning-up under the hood. I never tried Cocktail because Onyx is free.