If the greenscreens are well lit, you should be able to pull keys in either program. you may, however, want to buy a better keyer than what comes with either FCP or Motion.
While you’re at it, though, take a look at Shake. It’s now only $500, it comes with both Primatte & Keylight, and it has 1st rate tools for masking & rotoscoping.
Arnie
Now in post: Peristroika, a film by Slava Tsukerman
https://www.arniepix.com/blog