The major similiarity between the two poems is that they both speak of love and they both do it in pretty extravagant terms. Marlowe's shepherd talks about all the things that his love will enjoy if she will just be his. Marvell's speaker talks in hyperbole about how much he loves his love -- he talks about how, if he had time, he would love her since before Noah's flood.
The major difference is in the degree of explicitness in the two poems. Marlowe's is pretty staid -- come with me and be my love. But Marvell's is very sexual. He tells his love that, since they do not have all this time, they ought to start having sex now, while they are still young.
Both Andrew Marvell's "To His Coy Mistress" and Christopher Marlowe's "The Passionate Shepherd to His Love" are classics in their own right and have long been recognized as important contributions to English literature. More importantly, both poems are exceedingly famous examples of love poetry. However, while they are similar in this respect, it's also important to recognize that the way in which each poet expresses his love is subtly different.
First, the similarities: both Marvell and Marlow employ elegant (and often ornate) language in addressing their mistresses. Marvell, for instance, uses a lengthy first stanza to describe the vast ages of love and courtship that his mistress is worthy of, saying things such as "An hundred years should go to praise/ Thine eyes" (13-14). Marlow uses similarly hyperbolic and flowery language, telling his love that "I will make thee beds of Roses/ And a thousand fragrant posies" (9-10). Obviously neither poet will literally do either of these things; rather, both use over-the-top language to illustrate the depth of their affection.
It is important to recognize, however, that the poets have different attitudes toward their courtship. Marlowe, for instance, seems to have nearly no sense of urgency in discussing his courtship. Indeed, his relaxed, languorous writing style gives the reader the sense that he has no need to rush the relationship. Marvell, however, is obsessed with speed. "To His Coy Mistress" is essentially about a race against time and mortality, as the narrator pleads with his lover to have sex with him immediately before both he and she lose their precious youth. Therefore, while Marlowe's poem centers more on love for love's sake and expresses no worry at all, Marvell's poem is actually really about the terrors of mortality and old age, expressed within the context of a love poem.
There are many more interesting similarities and differences between these two poems. To read both poems and extensive contextual notes, check out the links below.