They are both awesome effects. They both require you to make your own gimmicked bill and they both have some awesome changes. Prophet lets you show 5 bills and change them with a snap. Greed lets you wave your hand and change a 1 into a 5, wave again and it turns into a 10, fold it again and have it magically turn into a 20, then wave the bill itself and have it turn into a quarter, 50 cent piece, or any other coin of your choosing.
Both are amazing. You can make a routine out of both of them even. Perform Prophet, do a bill switch and have your Greed bill out, then do Greed. I would highly recommend both of them. However, as a first effect I would probably go with Greed. Greed is just more of a fun effect in my opinion.
I highly recommend both though as they are both amazing effects.
-Doug