If this is expressed in terms of `24dB reduction' and `20dB reduction', then 24dB reduction is a greater amount of noise reduction than 20dB and, theoretically, superior.
Or have I misunderstood the question?
In my experience, noise reduction figures are measured in a rather idealistic test environment, and the true noise reduction will vary from head to head. It may not be worth paying a great deal of money for a notional extra 4dB reduction.