Choosing Your Game
Winning the bid is only half the decision. Once you've seen your hand, and the Skat if you picked it up, you still have to declare which of the three contracts you're actually playing.
Read the official rule on the three contract types.
The same Club Jack anchors a Farbspiel or a Grand; choose Null instead, and it is no trump at all, just a middling club above the Ten. The Seven beside it is the card Null actually wants.
Choosing your best game
The same hand often supports more than one contract. The right choice is whichever one your actual cards, not just your trump count, can realistically deliver the most value from.
Farbspiel
A suit is trump, led by the four Jacks. Suit games reward a hand with one clearly dominant suit: enough length and high cards in it to keep control of the trick.
Grand
Only the four Jacks are trump, nothing else. Grand's base value is high, so a hand with strong Jacks but no single dominant suit often scores better here than it would forcing a Farbspiel.
Null
No trumps at all, and you have to lose every single trick. Null rewards the opposite kind of hand: nothing high or dangerous enough to accidentally win with, rather than a hand built to dominate.