This question has been posed a few times before. The answer depends on the airflow you need, and on the specific fans.
If I compare 2 Panaflo 80Ls vs, at 5~7V they are quieter by a small margin than the best 120mm fan I have, a very good sample from an Antec Sonata. I have several of these Antec 120mm fans; this particular one is exceptionally quiet. There is a greater amount of lower frequency drone or hum from the 120mm fans that seems to get amplified by the air or panels a typical case; this happens much less with 80mm fans, even 2 of them.
In any case, the fundamentals of noise addition say that two 10 dBA (@1m) fans make 13 dBA. The Panaflo at 5V is somewhere around that 10 dBA level. No 120cm comes even vaguely close at 5V. On the other hand, the airflow/noise ratio of the 120mm fan probably always exceeds the 2 Panaflos, and at some point somewhere above 7V, depending on the Particular 120mm fan, it may become a better choice, not only for airflow but noise.
The gmJamez's Variation excel file
of fan spec calculations (airflow and noise at reduced voltages extrapolated from the manufacturers' 12V specs) in the article Choosing Fans for Quiet, High Airflow: A Scientific Approach
shows a 120mm NMB 4710NL B10
fan that is spec'd at 12V for:
59cfm * 1550rpm * 27dBA
Those numbers seem very optimistic to me, but if we take them at face value, the extrapolated 5V numbers are:
25cfm * 646rpm * 14dBA
That's certainly better cfm than you would get from two Panaflo 80Ls at 5V, and close enough in dBA to be probably indistinguishable, but until someone actually tries a few of the NMB fans, we won't know whether --
-- the acoustic signature is no more annoying,
-- the lower frequency humming will be more noticeable, and
-- whether received reality of these fans follows theporical calculations.
OFF-TOPIC: If you have not read the article Choosing Fans for Quiet, High Airflow: A Scientific Approach
, it is a compelling (almost obligatory) read for those studying fans, noise & cooling.
The gmJamez's Variation excel file is an updated, scientifically more accurate version of the original file linked in jverheul's quick response. That article has become a bit confusing; I need to tidy it up a bit for readers who never saw how it evolved, but it is definitely worth reading and studying carefully. A wealth of useful info there.