The reason you might want to go with a 45 size motor is that when you go to that second or third plane you will already have a motor that is strong enough for it.
The os or st or magnum's are all going to be good motor choices and wont be too heavy for the falcon.

The cheapest .25 and the most expensive 40-45 size motors will all fly the falcon just fine.

Luck, Dewayne