(a) Prosecution for any violation of law occurring prior to January 1, 1973, is not affected or abated by the Unified Code of Corrections. If the offense being prosecuted has not reached the sentencing stage or a final adjudication by January 1, 1973, then for purposes of sentencing the sentences under the Unified Code of Corrections apply if they are less than under the prior law upon which the prosecution was commenced.
     (b) Prosecution for any violation of law occurring before the effective date of this amendatory Act of 1977 is not affected or abated by this amendatory Act of 1977. If the defendant has not been sentenced before the effective date of this amendatory Act of 1977, he shall have the right to elect to be sentenced under the law as it existed at the time of his offense or under the law in effect on and after the effective date of this amendatory Act of 1977. If a sentence has been imposed before the effective date of this amendatory Act of 1977, the defendant shall not have the right of election even though his case has not been finally adjudicated on appeal; however, where eligible, he shall have the rights provided by Section 3-3-2.1 of this Code.