Both! You can use them independently, but they work great together.
Co-management enables you to concurrently manage a Windows 10 or later device with both Configuration Manager and Intune. You install the Configuration Manager client and enroll the device to Intune. The device communicates with both services. Configuration Manager policy controls which workloads you switch to Intune as the management authority. You can enable co-management and not use tenant attach to upload devices to the Microsoft Intune admin center.
Tenant attach sets up synchronization between your Configuration Manager site and your Intune tenant. Then you can see Configuration Manager devices in the Microsoft Intune admin center. This connection provides you with a single view for all devices that you manage with Microsoft Intune and Configuration Manager. This web-based console may be beneficial in some scenarios that don't want to use the full Configuration Manager console, such as with help desk staff. You can also benefit from insights in Endpoint analytics. You can configure the site to upload devices to the Microsoft Intune admin center and not enable co-management. However, those devices are still managed by ConfigMgr until you enable co-management.
For more information on tenant attach and how to configure it, see tenant attach.