Uh... which one? Choice is not obvious to me, and I think it would depend on the type of cancer.
Cancer. The worst types of it have the advantage of killing quickly. Alzheimer's destroys the self, and you survive a long time with it, leading to much more suffering, both to you (to the extent you continue to exist) and to your family.
Cancer is better than Alzheimer’s. There is no comparison. I wouldn’t wish Alzheimer’s or dementia on my worst enemy.
Seeing my grandmother go through dementia for 10 years made me incredibly angry that we don't have assisted suicide available for people.
People with the emotional and compassionate depth of a child are the ones keeping us from allowing people to die with dignity.
Cancer times a million
From multiple personal experiences, including both of my parents, dementia is a slow, horrible death where you are robbed of your dignity and end up dragging all of your relatives through a very long, very torturous hell. You will be drooling, pissing, and shitting yourself, all while slowly reverting back to a low IQ childhood mentality where you're very likely to have outbursts and verbally or physically attack the people around you. Your loved ones will be tormented, and if you don't have loved ones then if you're lucky you'll be tossed into a room and forgotten about by underpaid, overworked staff at some run-down nursing home. If you're not lucky you'll be laying in a gutter on the street until you die.