Essentially a prenuptial and postnuptial agreement are the same in the terms of the legal provisions that are covered. The only thing is a pre-nuptial agreement is more likely to hold up in court than a post-nuptial agreement. Courts have been reluctant to enforce the post-nuptial agreement because a spouse has a duty to take care of the other spouse. However, the trend in courts is that more than ever, judges enforce a post-nuptial agreement.
No attorney client relationship is created by virtue of this answer. Further, answers are for information only and should not be relied upon unless there has been a formal consultation. Many cases are fact specific and there is simply no way an attorney could possibly provide complete advice without all the relevant facts.