Rap vs. poetry

Most writings about the difference between rapping and poetry are merely an artificial opportunity to note the difference between Snoop Dogg and Edgar Allen Poe or some such mismatching. Of course, we should look at the two art forms with a fair, levelled comparison, or at least objectively.
Just as raps have a stereotypical subject matter, poetry could be thought of as being solely about crying, love and over-dramatic metaphor. Let's pluck a poem from the depths of deviantart.com:

"And the sun peeps over the horizon
where the sky meets earth
her rays stretch out and
caress Your face
i look down and marvel at Your
Perfection"
(Sic).

Of course, having been written by an unskilled poet with sub-standard grammar (that is, there is no significance of the grammar used in the poem), it's not really better than Snoop Dogg. If we compare it to a rap by a skilful rapper:

"Right before he pulled the trigger and ended her life,
He thought about the cocaine with the platinum and ice
And he felt strong standing along with his new brothers,
Cocked the Gat to her head, and pulled back the shirt cover,
But what he saw made him start to cringe and stutter,
Because he was staring into the eyes of his own mother.
She looked back at him and cried, cause he had forsaken her;
She cried more painfully, than when they were raping her.
His whole world stopped, he couldn't even contemplate;
His corruption had successfully changed his fate,
And he remembered how his mom used to come home late
Working hard for nothing, because now what was he worth?
He turned away from the woman that had once given him birth
And cried out to the sky because he was lonely and scared,
But only the devil responded, because God wasn't there
And right then he knew what it was to be empty and cold
And so he jumped off the roof and died with no soul."

The main ingredients of this particular verse are rhyme (both end-line and half-line), assonance, metaphor and raconteurship.
It is therefore established that there is no correlation between the two styles and the amount of skill involved, so while it is tempting to look at the pair subjectively, it is more advisable to look at them objectively.
Subject matter is the most commonly observed factor in the comparison between rapping and poetry, but this can be rebutted by looking at the mass of poetry from the 21^st century, including works by Robert Frost, Roger McGough or Carol Ann Duffy, whose works were often written about the culture of the working class (or "commoners" if you like) and sometimes even mentioned their violent ways (cf. Roger McGough's "Brick Wall" or perhaps Robert Frost's "Two Tramps in Mud Time"). Another commonly considered (fallacious) difference would be the application of slang, especially ebonic slang. This perpetuates the view of poetry as a "high art form"; however the poem "Half-Caste" by John Agard uses ebonic pronunciation of English words so it is a myth.
What with "rap" describing the vocal delivery of a rap, it is worth noticing the difference between the prosody, or vocal rhythm, of the verses in question. Compare this extract from "Imaginary Places", by the rapper known as Busdriver, including the pattern of stressed syllables, to the natural prosody of the phrase used (U signifying an unstressed syllable):

Rapped:
U    /      U     U     /      U      /         U     U    /
I'm just here to hold your hand when you die

Spoken:
U     /      /      U    /      U      /        /        U    /
I'm just here to hold your hand when you die

Artificial prosody is therefore one of the main features of rap, while poetry traditionally uses the natural pronunciation of words to shape a rhythm, or, more modernly and somewhat anarchically, ignores rhythm altogether. Observe these two extracts of poetry and their rhythms:

"Unthrifty loveliness, why dost thou spend
Upon thy self thy beauty's legacy?"

"Such a terrible term,
out of order,
as if to say,
that which does not follow the order,
is broken,
instead of different."

The use of the rhythm in the first quote is called iambic pentametre, a common tool used in poetry of the time, and the general study of poetic metre is a whole, individual branch of poetry. The second quote has no such thing.
The existence of artificial prosody in rap allows raps to be rapped at whatever speed the rapper desires, allowing for much competition in how quickly one can rap. Despite this apparent disregard for semantics, the substance of words is equally important in both art forms. This becomes obvious when seeing the reaction of onlookers of freestyle raps to certain, quotable phrases uttered by the participants, while this importance is apparent in poetry when witnessing televised performances of poems, whereby particularly important words are picked out and shown across the screen, highlighting their semantic substance.
Overall, it looks like a matter of opinion, but no, the difference between these two art forms is the same sort of difference between any two given art forms, even if one is used in a way that isn't to improve on the beauty of humanity or some such bollocks; it is not just an opportunity to ridicule Soulja Boi.