If any sheriff is eligible for retirement under any other county retirement act, he shall make a choice as to which act he desires to come under and shall so notify the county governing body within 60 days after he becomes eligible for any payments under this article. In the event that an elected official’s retirement system should hereafter be established by law, any official elected after the establishment of such retirement system who otherwise would have been covered by the provisions of this article shall automatically be subject to the provisions of such retirement system as a matter of law.