Should i get novocain (procaine) or "laughing gas" for a cavity filling?

Either: Some restoration are so small, you don't need anything. Others are large and deep enough that you should have the local anesthetic. Since your treating dentist knows how deep this cavity is, why don't you ask them?

Why Not Both?: The gas makes an unpleasant experience much more palatable. The shot makes it painless.

Pain free: Everyone has different pain thresholds and different preferences. For some, laughing gas makes the procedure quick, simple and painless. For others they prefer a local anesthetic alone. Many prefer both. Discuss this with your own dentist and he/she can help you decide what is best for you in each circumstance.