It depends. In my experience it is almost always good to pick the hard compound in the original gp3, because the tyre wear ratio is so absurd. But in Gp3-2000, you can use both compounds on almost all tracks with success.
The trick is to get your choice of tyre to fit into your style and chosen pit strategy for the track you are at. If you like going fast, like overtaking a lot, have a high amount of tyre wear, and generally like softer springers, go for the extra pits stops and the soft compound. If you like higher springs settings, are good at conserving your tyres and don't mind cruising behind the AI and just waiting for the right time to pounce, but also overtaking less, then go for the hard compound and fewer pit stops.
There really isn't a "right" and a "wrong" tyre choice. It depends on how you like your races and how you like the car...
It's only after we've lost everything, that we are free to do anything.