Microsoft has talked about the difference between Preview for developers and Windows Insiders aka Technical Preview.
Windows 10 Technical Preview for phones gives you a sneak peek at Windows 10 for phones. If you’re interested in getting the latest software updates to Windows Phone 8.1, use Developer Preview for Windows Phone 8.1 instead.
There are many missing features on Windows 10 for Phones preview that is available on Windows Phone 8.1. Check the list below and decide whether you want to go for Windows 10 preview.
Messaging is currently missing a number of features you might use in Windows Phone 8.1, including dual-SIM support, search, message drafts, sharing voice notes and ringtones, Bi-Di language support, international assisted dialing, spam filter, viewing all group recipients, reminders and quiet hours through Cortana, multiple attachments, and GSM support. If you use these features regularly, you’ll want to wait to install the preview until we get them into a future build.
Cortana is only available in English (United States) for phones. In other locales, you’ll get the standard search experience.
The Rooms feature is not available in the preview. You’ll still be able to get to your photo albums, calendars, and notes, either on the web or by using other apps on your phone. You won’t be able to see your chat history or send or receive any new chats. If you have important info in your Rooms chats that you want to keep, save a copy to a different device before updating your phone to Technical Preview. For more info, take a look at the Rooms FAQ.
Some text in the preview will not yet have been translated into all available languages and will only appear in English. This text will be translated into all available languages by the time the final product is available.
The Phone app doesn’t have Dual SIM support or spam filtering in the preview.
Accessibility features are still being developed for Windows 10, so they aren’t fully functional in Technical Preview. If you depend on features like Narrator while browsing the web, you may want to wait to update your phone.