I would say the Rehabilitation route may be slightly an easier route than demonstrating extreme hardship on the proviso that the conviction (CIMIT) is over 15 years. In order to demonstrate Rehabilitation, the following factors would be necessary i.e. no further criminal convictions, steady employment, demonstrate that the individual has volunteered in the community/church etc, character letters from family and friends acknowledging that they are aware of the individuals conviction as well as stating that the individual in their opinion is now trustworthy, hard working, dependable, honest and in their opinion is rehabilitated etc. Additional letters from the volunteering organization/ employer etc would also support rehabilitation. In addition the individual is also required to provide a personal statement explaining the events that led to the conviction and in doing show also show great remorse and ask for forgiveness. Additional factors towards showing rehabilitation may also include but are not limited to any non immigrant waivers that may have been approved, any trips to the USA and returning back to home country in a timely manner under the visa/waiver conditions etc.
Hope this helps