Which is more serious injury: a torn patella tendon or a torn acl?

Both. They are both serious injuries. A torn patellar tendon results in a knee that doesnt function. An ACL tear results in an unstable knee, that can give out on you. The patellar tendon always requires surgical repair. The ACL usually requires surgical repair.
Both require surgery. If you are describing a completely torn patella tendon (not just a strain) vs. A torn acl then it is likely that the torn patella tendon would be considered the worst of the two. A torn patella tendon causes a person to be unable to bear weight or extend the knee. Most people are actually able to stand and walk w/o many issues when it comes to a torn acl.
Both are serious . I do believe the result from repair them the patellar tendon give better result.

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