Hiya! Welcome to the forum!
Well, as you say it does depend to some extent on other factors such as your case etc. But i would say that almost without doubt 2 fans @ 5 or 7V will be the best solution.
Lets say that you were using M1As, and a L1A represented an M1A at 7V (i think this is valid in order to explain this?). Heres their stats,
M1A: 28dB 32CFM
L1A: 21dB 24CFM
so 1 M1A = 28dB and 32CFM
but 2 7V M1As (L1A) = 2x 21dB and 2x 24CFM = 24dB and 48CFM.
(You know dB are logarithmic?)
so two fans at 7V is less than half as loud and gives 50% more airflow...
If your unsure about the dB sums just say and ill elaborate