I would recommend that you run a test with both keyers and see which one you like best. Personally, I think the FCP Chroma Keyer works really well for a lot of jobs. But it depends on the key. Best of luck and hope this helps.
Aw, jeez, if you're going to go to all the trouble to pull a key in After Effects, do the whole shot -- foreground AND background in AE! Quit screwin' around with this silly FCP-thinking!
In AE you can:
Color grade so that foreground and background look better together
Fake shadows to better integrate the subject with the background
Further sell the fact that subject and background belong together by creating light wrap
Time out motion graphics so it matches what the subject is doing and saying
Render in the codec of your FCP edit timeline--no rendering needed, no fooling with ProRes 4444
Now, if you're a big fan of the weatherman-with-radar-behind-him look, then pulling a key in AE and putting the alpha-channeled footage over any old background is just the thing for you. If you'd like to take a bold step out of the nineties, try doing the whole shot in AE.
There are those who would counter by saying, "Well, if I do it your way, I lose the flexibility to change backgrounds if I want."
I say, "Baloney! You're creating an effects shot, and if you're too disorganized to have overlooked the background until now, you shouldn't be doing the shot in the first place."
Sr. Promotion Producer
KCRG-TV (ABC) Cedar Rapids, IA