Both slots and sports betting can be highly addictive but slots are generally considered more to cause addiction due to their fast continuous play and unpredictable nature which creates quick reward cycle that can lead to people constantly trying to win back money.
However sports betting can also be very addictive especially for those who enjoy analyzing games or are emotionally invested in sport as desire to use their skills and follow their favorite teams can be very strong. Rise of live betting has also made sports betting quicker similar to slots but ultimately most addictive form of gambling can differ for each person.
Reading your answer, I first thought that sports betting and slot machines have one very obvious common feature. Look: slot machines keep the player in suspense, because each subsequent spin can lead to a win. But at the same time, in betting, when a person who has placed a bet sits and watches a football match, the fate of his money is decided every second. After all, a kick at a ball flying right into the goal is a matter of milliseconds.