It's a balancing act. 120mm fans don't have higher cfm than 140mm (assuming identical fans) because it pushes air through a smaller space. The only time the smaller fan has higher cfm is when the rpm exceeds the larger fans by more than an equitable amount. Any 120mm at 1000rpm has lower cfm than the identical 140mm at 1000rpm. But those same fans will push the same cfm if the 140mm is only 750rpm.
Optimal case airflow for a mid-tower ATX is 2x intakes and 2x exhaust. More fans don't add much overall. With 3x 120mm you'll get pretty equitable airflow to 2x 140mm as the center fan has much of its draw stolen by the upper/lower fans, negating a portion of its ability.
Fans are different. Some fans have a very tight exhaust cone, like the Noctua NF-F12, it's focus flow design put all the air straight out from the fan. That makes for maximum 'feelable' breeze. Other case fans have a very broad cone, closer to 120° or more, so while they may put out identical cfm, identical amount of air, it's spread so wide it 'feels' like its not doing much of anything. There's no hard breeze to feel.
I agree, the beQuiet silent wings are a good choice.
For good flow, you only need to move air IN to replace the vacuum created by the draw from the exhaust fans. It doesn't have to be huge amounts or hard breezes, it just has to move efficiently. Air goes in, picks up heat, goes out. All in one smooth motion. If that air is moving, you could add 4 fans in front and 4 fans on top, you'll not pick up any more heat than 2x2 fans, all you'll do is add noise and necessity of supplying power.
2x 140mm intake, 2x 140mm exhaust is about perfect or any combination that adds up as equitable, so you could swap 2x 140mm for 3x 120mm or 1x 200mm. All about the same.
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