In Marlowe's poem the shepherd is the protagonist. He is the one who attempts to persuade his lover to come away with him for a life of Edenic rural bliss. In Raleigh's rejoinder, however, the roles are reversed. Now it is the nymph's turn to point out the practical flaws in the shepherd's elaborate scheme. For one thing, the swain is mortal, while the nymph immortal. No matter how pretty the shepherd and the natural world may be at this precise moment, in due course they will both fade and decay. As an immortal, the nymph can spend all eternity surrounded by beauty that will never die. So all things considered, the shepherd's generous offer doesn't really amount to all that much.
The object of the shepherd's love and affection is a passive character. We never get to know anything about her, or find out what she wants out of a relationship. Raleigh's nymph, on the other hand, is an assertive character, leaving the hapless swain in little doubt as to what she wants out of life. (Or more accurately speaking, eternity.)