Should I use nRFConnect SDK or nRF5 SDK with the Laird BL65x modules?

Nordic nRFConnect SDK, also known as NCS is based on the Zephyr RTOS with support for Laird modules based on nRF52 and nRF53

Nordic nRF5 SDK is a legacy bare metal SDK with support for Laird modules based on nRF51 and nRF52

You can choose either SDK but need to know that nRF5 SDK will not be supporting new features going forward. So if starting a new BL65x or BL5340 project then it is recomended to choose nRFConnect SDK.