Both poems, "To His Coy Mistress" and "The Definition of Love" are metaphysical, applying some abstract idea, in this case, love, to something physical or tangible. In "To His Coy Mistress," Marvell compares the speaker's love to a vegetable.
In "To His Coy Mistress," the theme is carpe diem, sieze the day. The poet urges his lady to take advantage of the time they may have together, because "time's wing'd chariot" races along quickly. The speaker wishes that they could stay young forever and devote themselves to love completely. Since eternal youth is not an option, he encourages his beloved to be "like amorous birds of prey, Rather at once our time devour."
"The Definition of Love" is also about love, but unlike "To His Coy Mistress," this poem concerns unrequited love. Here, Marvell's heart aches for a love so divine that it could only be revealed by his "magnanimous despair." In this particular poem, the speaker's love is for someone unattainable; because this love may never be fulfilled, it can live on indefinitely "Unless the giddy heaven fall,
And earth some new convulsion tear" to bring the two lovers together.
The main difference between these two poems is the two speakers' approach to love. The speaker of "To His Coy Mistress" approaches love confidently, pleading with his lover to seize the moment, wheares the speaker from "The Definition of Love" feels that disappointment and rejection from his love is almost a foregone conclusion.