Posts and Pages have qualities that distinguish them from one another. These qualities will help you determine which is best for your use.
Time Sensitive
Posts are generally based on time. They are usually displayed in reverse chronological order and their URLs incorporate the day they were published. Pages are more for static content. They are specifically designed to exist from year to year and are not time sensitive.
RSS
Posts fit best into RSS since they are based on time. However, since pages are not based on time, they are usually not included in RSS.
Commenting
Posts are meant to generate conversations which is usually why they have comments and can easily be shared. Typically, pages are standard information such as an About page or Contact US and in most cases, you would not want comments.
Organization
As mentioned before, posts are organized by time but they can also be organized by categories and tags. Pages are organized hierarchically with unifying topic and/or parent page creating their relationship to other pages.