This choice depends on the final look you want and your future needs. In our experience:
Slim Metal Clips: These provide the best seamless finish, allowing boards to be fitted closer together with minimal gaps. Because of this, close attention must be paid to the 3 mm side-to-side expansion gap during installation. The main drawback is that if you need to lift a board from the middle of the deck later, you may have to remove several boards to access it.
Plastic T-Clips: These create a slightly wider, more traditional gap, which allows for better expansion and drainage. Crucially, they allow you to easily replace a single board anywhere on your deck after installation, as the screws are often accessible.