In these two poems, imagery seems to be most important.
The images in both poems are very powerful. Randall uses some rather difficult images to describe the speaker's decaying body:
Even though I be a clot, an aching clench,
a stub, a stump, a butt, a scab, a knob,
a screaming pain, a putrefying stench,
still let me live, so long as life shall throb.
Here, the speaker dictates the truth of death. His imagery (and diction and language) illustrate that he knows how bad it will be, if someone is questioning whether or not to kill him out of mercy. He knows what it will be like and is able to support his cause that he wants to continue under the direction of his own mind and body.
The speaker in Thomas's poem also uses imagery to fight against death:
Wild men who caught and sang the sun in flight,
And learn, too late, they grieved it on its way,
Do not go gentle into that good night.
The important image here is the sun in flight. We can envision the wild men and what they are like, "sing[ing] the sun in flight."