Size and reach: The size and reach of the SSP are important as they determine the quantity and quality of advertisers available for your advertising spaces. Larger SSPs typically have more advertisers and a wider variety of available ad formats.
Average CPM: A high CPM indicates that the SSP offers higher-quality ads that are attractive to advertisers.
Geographic footprint: Consider the geographical regions in which the SSP operates.
Segmentation options: Look for detailed and precise segmentation options to personalize ads based on user interests.
Access to exclusive inventory: Some SSPs provide access to exclusive inventory, allowing you to differentiate yourself from competitors and attract unique advertisers.
Variety of formats: It’s important for the SSP to offer a wide range of ad formats, including video, display, native, and others.
Costs: Costs may vary depending on the size and reach of the SSP, as well as additional services they offer.
Interface usability and reporting capabilities.

In addition, key factors to consider include the technical support provided by the SSP. It should be responsive and accessible to address technical issues, minimize downtime, and maximize revenue opportunities.