CFM vs Diameter

What do you think would be more effective auxiliary electric fan, a
14” @1800cfm or 16” @1600cfm?

Confirm you are adding this to an existing mechanical fan set-up? Is your car overheating with the stock set-up? Are these pusher fans mounted to the front of the radiator?

Both those CFM ratings are pretty low, but the stock electrical system could probably handle them if you use relays.