Just want to get some opinions on this.

Suppose that you had a case where you could mount either a 120mm or 140mm fan as an exhaust. What size would you choose? Why? And which fan would it be?

I've also thought that for exhaust, any slow rpm fan would do a decent job. But wouldn't a larger, 140mm case be better because it's spinning at a lower rpm vs. a 120mm that has to spin faster for the same rpm?

Yes, all things being equal it's better to have a bigger fan. But it's often not quite as easy to find a really good 140mm fan at a good price and with local availability as it is with the 120mm form. The difference in performance isn't necessarily too substantial either, but when mounting a 120mm fan in a position that also takes a larger 140mm fan it's a good idea to block off the slack area of openings to avoid having any localised circulation of air in and out of the same vent. A little extra work.

I only prefer 120 mm because of the vast availability (and therefore the lower price). Otherwise I'd choose a 140 mm fan in a heartbeat. I do have one concern for 140 mm however, and that is fan starting voltage. I want my computer dead-silent and therefore run my 120 mm fans fans at 3-400 rpm (lower than most starting voltages of many fans). Perhaps the greater mass of 140 mm fans will increase the starting voltage even further. I have a clever fan controlling system with my NZXT H2 and the stock fans, so that the fans start with a high voltage before I dial down the rpm to below starting-voltage. But I do appreciate fans with very low starting voltages (Slipstreams for example) since I want these to be run by my mo-bo (but perhaps modern motherboards kick-start fans with high voltage to ensure that they start).

I've also wondered about this, because while a few popular 140mm fans has mounting holes that fits a 120mm fanmount - still the actual hole it blows/sucks through is maybe/often/easily to narrow (not 140mm) and will cause turbulence & noise?? I then also think, is the circumference of the airflow strongest because of the greater speed of the fanblades at the tips, hence more turbulence?

My thoughts exactly

The bigger fan should have more airflow at the same RPM. If you lower the speed of the bigger fan to match that of the smaller one, its air flow will drop to that of the smaller one roughly.
The starting voltage is a non-issue if you connect it to a fan controller with a capacitor to jumpstart the fan, that's a pretty cheap solution too.