Which characteristic best describes how nuclear decay differs from nuclear fission?

Nuclear decay is spontaneous.

Nuclear fission does occur spontaneously but at a very low level in fissionable materials. The main difference between fission and nuclear decay is that fission can occur by neutron bombardment. And when the neutron source is also other fissioning atoms, a chain reaction can may happen.

No chain reaction is possible with nuclear decay.

That would be a much better characteristic to look at.