Zigbee2MQTT supports more devices, and has better support for some devices that ZHA only has basic support. For example I have some vibration sensors which ZHA supports for basic vibration detection, but Zigbee2MQTT can set vibration sensitivity levels as well as detect vibration.

Zigbee2MQTT also has a more advanced interface with more configuration options, displays more information, and has better network visualisation.

Another advantage is that Zigbee2MQTT can be run on a different system from Home Assistant. That way the Zigbee coordinator can be placed in optimal location of the home, while the Home Assistant can be located elsewhere.

If you are already comfortable with using ZHA I there isn't much need to switch, unless you need support for devices only supported in Zigbee2MQTT or really need to relocate your Zigbee coordinator. Migrating a whole Zigbee network can be a bit of a pain. If you are curious about it you could always buy second Zigbee coordinator and run both.