Okay so simply put Preview for Developers was a program to get the latest Windows Phone 8.1 updates before your carriers or device makers pushed the update to your device. It was finished and stable software.
With Windows Insiders its different. The update that you get in this program is not finished. Its still under construction and might not be stable and have many bugs.
For further clarification please visit this forum thread.
Still have questions? Put it in the comments down below and I will try to answer each and everyone of you.