106 Miles to Chicago

XMLHttpRequest vs. IFrame

Good overview of the best time to use each.

This leads to a good rule of thumb. If you're updating the page with new or updated information, you most likely don't want a history event. So, use XMLHttpRequest. On the other hand, if you're working in resposne to a user request, it would often be better to use the IFrame approach in order to preserve back button behavior.