The IC-FL45 is a CW Narrow filter for the 9 Mhz IF, and has a bandwidth of 500 Hz. The IC-EX203 is an active Audio Filter which uses an IC op-amp (and supporting circuitry) as the AF filtering element. The EX-203 has a bandwidth of 300 Hz.
The advantage, of course, goes to the IF filter, as close-in QRM will be attenuated by the filter's stopband rejection, whereas with the AF filter it is not. While you won't necessarily hear the close-in QRM with just the AF filter, the receiver front end will, and you will see it on the S-Meter. The front end may be de-sensed by strong adjacent signals using just the AF filter, and AGC action could cause a weaker station you are working to be buried.
Using both the IF filter and the AF filter is a great combo - I have both in my IC-730, making CW very enjoyable.
73, Frank W3UHF www.icomclassic.com