It is important to know that the financial benefits that Google provides are the same for every CSS partner. The service level, fee models and USPs of each partner are the biggest differentiators.
Another difference to consider is the type of CSS model, whether that is using a CPC model to enhance your Shopping performance, usually on a fixed monthly subscription fee, or working with a CPA partner on the affiliate model to place ads along side your own Shopping activity charging commission on an acquisition basis. You can also work with partners who offer different types of service level from self service to hybrid support or a fully managed service.