Fitting a cowling makes a large difference. My fan previously would run most of the time in traffic and would rarely switch off which indicated that the temperature was not being reduce sufficiently to the cut off point.
It did reduce the temperature but then maintained the same temperature which never made me comfortable.

A pusher fan is 25% less efficient than a puller.

So I installed a cowling. Which an immediate difference. This now really pulls air and the temperature is pulled down to the cut off point.

I worked out the area difference of a 16 inch fan

area A= Pir = Pi8 2 = 201.06 sq inch

With the side tanks the radiator fine area is 18" x 21"= 378 sq inch

As a percentage area you are increasing the pull surface area by 88%.

Working on 2 x12" fans

area A= Pir = (Pi6 2)2 = (113)2 = 226 sq inch

So two 12" fans has a 10% greater area than a 16" fan but two 12" might have less CFM than a 16" fan which can be rated at 3000cfm. A straight bladed fan will be nosier but shift more air than a spiral bladed fan.

I have a twin speed 16 inch fan set at two different temperatures. I find it never kicked in on the fast speed so did not bother two wire it for the fast speed in the end.

1. Fit a 16 inch
2. Fit a cowling for great surface area
3. Puller fan configuration
4. Straight bladed will shift more air but check CFM rating over a spiral bladed fan
5. Check motors wattage rating. The higher the rating the more powerful the motor will be. Usually 140 watts rating
6. The fans RPM . This is not always stated.

I have hear of some 12" twin fans set ups being very poor fans which are sold on ebay imported from China. Avoid these.