Basically, all you ever need to know about fans is:
1/. martinsliquidlab
2/. Buy the Arctic P12 PWM fan.
At least, I'm 'reliably informed' that's all you need to know, since it gets trotted out so often...
While there is some vague truth to the above, the whole story and the bigger picture is often ignored. What Martin's Liquid Lab actually says is that the CFM and mm/H2O stats are not the
only thing that defines how good a fan is... and while they
do give a good indication to start with and are a useful comparison point, they don't always have precisely mapped curves that
may give a fuller picture of what you're dealing with.
He also mentions "Real World" performance, which he decides is 1500rpm... That's mid-range for some fans, but actually
beyond max-range for some others...
Even with my own 360 rad, each fan (all three same make/model, of course) performs slightly differently due to the location in the case and the slight variances in manufacturing. In general they are pretty silent until I start nudging 2000rpm, yet another fan of the exact same make/model put on my rear exhaust grille (with larger holes and far less restriction) makes an absolute racket above 1200. Another two at the front, again of the very same fans, perform better because the rad is thinner but are noisy above 1800 because the air spreads sooner and reverberates around the (empty) drive bay.
But this "Real World" stuff is at 1500rpm and each of my fan groups runs at different speeds, depending on the situation. When I ran different models of fan, the difference between rpms was even wider, and when I used rads with different 'Fins Per Inch' counts it was wider still... and for the record, I ran all my Noctuas and Gentle Typhoons at max revs all the time.
But all that aside, he does point out that even these lovely curves are only done in the "open air" with no restrictions, so again not the full picture.
What matters most is how any given fan performs when paired with a particular radiator, in a particular case, with whatever particular components it might have inside. There are so many unique factors that affect your fan that it's pretty impossible to predict.... otherwise this would be an exact science and all fans would be awesome.
You want a decent amount of airflow, with a decent static pressure to force it through, all with noise low enough for your particular preferences, and so it's really finding yourself a balance between all three of these measures. Most of the time it's a case of just trying different fans until you're happy.
But to answer your question:
1/. CFM is how much air the fan can move.
2/. mm/H2O is how much force it uses to push that airflow through a restriction.
It doesn't matter how high CFM is, if it doesn't also have enough mm/H2O push it. High CFM is like a strongman who can wrap their arms round the biggest Atlas stone, but pairing that with low mm/H2O is the equivalent of always skipping Leg Day!
So in short, a high CFM is good, but only if you have a high enough mm/H2O to balance it. Too high on either can cause a compression stall and actually prevent the fan from performing as well as it should. But again, try it and see... Same for fans is Push or Pull. There
should be very little difference in noise and performance between them (except that Pull traps less dust), but it's worth trying both out. Same for whether you have them set to intake or exhaust.
or just get silverstone down to top airflow case and sorted