What is a good element to compare/contrast in the poems "The Flea" by Donne and "Porphyria's Lovers" by Browning?A most important point relating to...

I would look at the comparison, or metaphor, created
between sex and violence in "The Flea." First, the flea bites both of them. Their blood
is intermixed in the flea and this is the speaker’s rationalization that they should be
intimate since their fluids are already together. When she kills the flea, the speaker
indicates that this is a murder of three: the flea and the two of them. When the flea
dies, the speaker concludes that since he and she have not been diminished in any way,
that their love is also undiminished. Therefore, premarital or merely physical intimacy
won’t diminish their love or their lives. There is also the indication that this
sacrifice of the flea has proven that any intimate act is not sacrilegious. This is all
part of the speaker’s rationalization.

In “Porphyria’s
Lover,” the speaker strangles Porphyria in order to immortally preserve their love. The
speaker in “The Flea” goes through all these rationalizations to convince his lover of
his objective. He even uses the violent act of killing the flea as evidence for his
position. Similarly, at the end of “Porphyria’s Lover,” the speaker notes that “God has
not said a word!” He says that the lack of God's retribution proves his actions are not
to be condemned. So, just like the squashed flea, this violence is not to be
condemned.

You could go with the metaphor angle. The
violence of the flea bites, the flea’s death and Porphyria’s strangulation are all used
as evidence of the purity of love. In other words, both speakers use these violent acts
to prove and sustain their love. Clearly, they are deluded by their own passions and
they will use these metaphors of love to prove the rightness of their love and
desire.