Which would be better?

The 59-3370 9" dia. 5" tall element.
Or
The 59-3414 14" dia. 4" tall element.

There won't be any performance gains with either over each other or over a stock flame arrestor.

Either would be fine. I would go with the 9" to keep more room on the sides. As far as performance gains, well clean air is good, I don't believe in trusting a window screen.

I actually contacted K&N and discovered that the 9" wouldn't flow enough air for my motor. Check the airflow on your flame arrestor, you may be surprised as to how little they flow also. I used the 14" with a velocity stack style housing to smooth the flow into the carb. Fortunately for me, Velocity has a cold air intake built into the rear deck, I just had to open it up a little and it actually fit inside better with the velocity stack style housing.

Good advice ThisisLivin. This is what I did on my last build with a pair of 472's.