The answer to this question depends on the type of application you have.
If your software is a native application, i.e. does not work through a web browser, or does not have communication with a backend cloud service, then the Polling API will have to be used in order to avoid being blocked by the end users' firewall. With the Polling API, as it uses standard HTTPS to communicate with our cloud servers, no firewall configuration changes should be necessary.
However, if your software is SaaS (software as a service) and works through a backend web service, the Webhook API is recommended to reduce traffic and to make sure that transaction data is recorded even if there is a failure on the local device.
A combination of both is also possible. The Polling API for the local app to check for transaction status, and the Webhook API to report to an online server for backend data storage and redundancy, or a data translation service such as Zapier.
Still unsure which you need to use, or which might work best with your current platform? Open a ticket and let us know! Our staff can discuss your needs and assist in bringing easy EMV processing to your current product or platform.