Here's a little advice: if you're really stuck between synonyms, have a look at the etymology. I normally use this site: www.etymonline.com "Rapacious" clearly comes from Latin - note the suffix - so it has a formal feeling to it. Literally, it means "grasping/grabbing". "Greedy" has an Old English origin and has a common feel (again, note the suffix). The origin behind this word is "hunger/want".

Both are pretty much the same. The main difference is that 'greedy' is more of a selfish desire of something. Rapacious: 1) excessively grasping or covetous. 2) living on prey 3) very eager or greedy for food, satisfaction, or gratification. Greedy: 1) having a strong desire for food or drink . 2) having or showing a selfish desire for wealth and possessions. You can see that the only difference is the word 'Selfish' which makes it more of a negative word to use. However, animals like coyotes, foxes, and bobcats can be Rapacious because they live on prey.

Both are synonyms, but rapacious means more violent; being voracious or avaricious.