I would generally agree with TOGA Descent but I have only first hand experience with Rishworth and know PARC only from second hand information.
As long as they need you evreything is fine, I think this is valid for both of them. I always received my payment ontime it was never late but there were some issues concerning banking fees and also the medical insurance offered was not the best one. The rest was just fine.
Keep in mind that the contract market is a tough field for the agencies. They all want to make money that's their business goal and they have to fight to get a contract and to keep it, so they must keep their customers, the airlines, happy. With this kept in your mind you can truly understand that at the end you might end up between the lines and left alone. This is valid for all of them and not limited to the above mentioned companies.