I would say they are both equally difficult... poetry flows out easier, less restriction on form, rhyming is not necessary, but you have to be able to Move the reader in a profound way....

Rap - it's easy to make something rhyme or say something witty, but to write it in a way that allows for excellent delivery and impact takes talent... plus you have to be able to turn that written song into something that sounds good...(unless you are talking about text only, which in my opinion doesn't compare to good poetry no matter how well a verse is written).

but the bottom line is, RAP IS POETRY