The simple difference, apart from academic requirements, lies in the type of work they do.
CEng: Tend to really move technology forward, working on complex research, design and development of whole systems. Highly analytical, innovative and creative. Always looking to maximize efficiency. The person who leads the whole design of a new jet engine is likely to be a CEng for example. Leaders of complex engineering activities.
IEng: Tend to maintain, build or carry out minor or less complex development work on 'as-is' product. Sometimes they might try to protect the ways things are currently done, rather than always seeking to maximise efficiency (don't shoot me, just my experience of them). Managers of some types of engineering activities. Often they will report to a CEng.