Whenever possible - I use a TV. But you need a Switch Pro controller to play on TV (The default Switch Joycon controllers are no fun to play on TV). TV offers some huge advantages. For example, the resolution is greater on TV and game might have more FPS too on TV Mode. Plus the Switch Pro Controller is one of the absolute best controllers in the market and for quite a bit of time it was the No.#1 Controller in the market (From what I heard, Switch Pro Controller is the previous version of the Dualsense and shares many features with it).
Advantage of handheld is portability of course. It has it's drawbacks, like reduced FPS etc. but it's still quite good. I use it if I'm travelling, or just in the mood to use it as a handheld.