Both of these poems firmly fit into the "carpe diem" or seize the day category of poems. These poems are a call to live life to its fullest right now. By reminding the listener of the fleeting nature of mortality, the speaker hoped to urge them to love and live life to the fullest now, rather than wait until tomorrow. This of course is the most obvioius theme of both of these poems. Note the imperative beginning of Marlowe's verse:
Come live with me, and be my love...
Both speakers seek to persuade their mistress to engage in a realationship with them now, rather than waiting until tomorrow.
However, if there is a difference in theme it is the way that Marlowe's poem ignores the threat of death and decay and focuses just on the country, pastoral pleasures that the speaker will create for his mistress. We are presented with lavish images of the "beds of roses" and embroidered clothing that he will weave himself for his loved one. Country sights and sounds dominate as the speaker tries to tempt his lover with all the pastoral delights his imagination can muster.
In contrast, "To His Coy Mistress" includes lots of reference to the fate that awaits us all, and seeks to use this fate as a spur to convince his lover to engage in a relationship with him:
Thy beauty shall no more be found,
Nor, in thy marble vault, shall sound
My echoing song; then worms shall try
That long-preserved virginity,
And your quaint honour turn to dust...
The presence of time, in his "winged chariot," is never far from this poem, and thus there is much more of a focus on the ephemeral nature of human existence and how quickly beauty fades and we all die. Better, then, to live and love today before it is too late...