Well, whose car is it, anyhow?
If it's the radiator company's car, do what they say.
If it's your car do what you say.
Sometimes free advice is worth what you pay for it.
There is NEVER an advantage for a radiator manufacturer or seller to tell you to use a smaller one. Think about it - If for some reason it (the smaller one that they said was OK) doesn't work, it's their fault. And, if they tell you that you need a big one, and you buy the big one, it is more of a $ale.

Radiators are just heat exchangers, they're not magic. Engines are nothing special either. The fuel energy in - the work the engine does = the heat energy out. Some is transferred out of the engine by the block, some goes out the exhaust pipe, the rest goes out through the cooling system. So if your two engines have approximately the same thermal efficiency, each will move heat to the cooling system in proportion to the power output of the engine. If the power output is the same, the cooling load should be very close to the same.

Now if you are pulling 20,000 pounds up a hill as fast as you can, a 500 hp engine may put 2.5 times as much waste heat out the cooling system as a 200 hp engine. Keep in mind both vehicles are accelerating as hard as they can (maximum engine output).

R.