I won't write your essay for you, but I will help you with some ideas.  You should begin by noting what elements of these two poems are similar and how they are different.  Let's start with the differences.  One is a sonnet and one is not, so there is a definite format issue.  Both speakers are male, and both have a love for which they would do anything.  However, in "Valediction", the speaker is telling his love not to mourn the fact that he is leaving her temporarily (work or business trip, maybe?)...mourning and crying tempests of tears would profane their love and make it less than it is.  Their love is stronger for their absence because it is more than just the physical...it is emotional, physical, spiritual, and elemental.  It transcends the boundaries of the earth like the comets and other things in the universe we don't see, therefore we don't fear them.  The speaker tells her to have faith and be calm in his leaving, like the man who dies who knows he is heading for Heaven.  The woman is the part of the speaker that keeps him grounded, like the stationary part of a compass which is planted in the paper while the pencil takes off to make its circle.  The speaker is the pencil, and he will always come "home" because she completes him.

In "Sonnet 29", the speaker's love also keeps him grounded.  She is the one who brings him back from the brink of destruction and complete self loathing when the world treats him unkindly.  He is wishing he had more luck, was better looking, had more talents and skills, and in a deep depression where he no longer enjoys the things he used to enjoy doing.  Luckily, just before he goes over the edge, he just happens to think of his love, and then he "wouldn't change places with Kings".  The thought of her love brings him such joy that nothing the world can throw at him could possibly bring him down.

So, in "Valediction" the woman is sad about her man's leaving and he is giving her the pep talk.  Donne also uses metaphorical conceits in his poem to make his point.  His comparisons use unlikely subjects--one is the comparison of his love and the compass I have already discussed.  The other is the comparison of their love and "gold to airy thinness beat"--no matter how far it stretches, it doesn't break.  Just like their love, no matter how far away he travels, they are still always connected.  In "Sonnet 29", the speaker is in the downy-dumps and the thought of his love brings him the pep talk.  Both works include a woman who is the center of the speaker's life.  She is the sunshine on a cloudy day for both speakers.

Good Luck!  When you've gotten part of your essay written, consider using the essay lab here at enotes for some feedback.  The link is below.