I have had mixed experiences with both shipping methods. There have been a few cases where something I sent through first class international reached its destination within a week, while something I sent through priority mail international took 2-3 weeks. My best guess is that the latter got stuck in customs.
There have been other times when I sent something through first class international, and it took two months to reach its destination. This brings me to the conclusion that it depends on what shipping method you use, where you are sending the item, and what time of year you are sending it.