I'm not sure what is included in Logic Express, but I do know that Logic Pro comes with a pretty extensive plug-in library (70 is the current count last time I checked), and peoeple seem to be very happy with the included software instruments.
Obviously some things will be take out in Express, but I'm guessing it's still a step up from FL Studio. My advice would be go do a search on google for something like "Logic Express vs FL Studio", and see what comes up. Since many of the artists here use FL exclusively, you're going to get a biased opinion, especially considering many of these artists may not have even touched Logic.
There is no argument, however, that Logic's audio production features far surpass FL's. Logic is meant as an all-around music production kit much like Cubase and Sonar, so if you're going to be using both audio and midi on a regular basis, go with Logic.