Why do we always discuss exact measures in emotional terms?
The 320 cabin is 7 inches wider than the 737. In 3x3 configuration there are four places where shoulder/elbows meet. With the same aisle width, if the shoulder/elbow separation is 2 inches on the 737, then it is 2 plus 7/4 = 3.75 inches on the 320. That's quite simple math.
On my emotional scale: With 3x3 config. the 320 is tight, the 737 is cramped.
Some airlines have figured it, and at least in business class configured their 737s 2x3. That of course beats everything.
But what does it matter? Both planes are made for flights lasting no more than a few hours. The tighter shoulder separation is more disturbing on a transatlatic flight on a 3x3 configured 757 which shares the 737 (727, 707) cabin width.
And after all legroom is a far more important comfort parameter. And totally airline decision dependant.
My only long flight on a Boeing 3x3 configured narrow body plane was on a 707 fifteen years ago. We were 11 pax on board. I put up the armrests and slept on 3 seats, quite comfu...
Regards, Preben Norholm