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.
What do you use them for and do you understand what they're doing? I'd argue that it's unnecessary for the average user to have these apps installed. 🙂