Need some opinions for a GUI editor

[Flamaros]

Hi,

I and a friend are developing a GUI library, and now our script 
engine is ready to start a prototype (but far to be finished). We 
think to try to create a GUI editor based on our library.
In this way, we'll see which features are need.

My concern is about how the editor have to works, we see two 
different ways to do it :
1) Classic editor external to the user applications
  a) Good :
     - Lightweight (easy to deploy and test)
     - No need to modify application code
     - Stable due to isolation of application
     - Real-time edition but limited on one view (bad to preview 
menus transitions)
  b) Bad :
     - Limited, plugins needed to extend editor components and his 
knowledge of application (can't predict size of unknown 
application specific items)

2) Integrated editor (launch with the user application in a 
second Window)
  a) Good :
     - Preview is the final result with real data
     - All application components accessible to the editor without 
complex plugin system (in this way all editors components will be 
well placed in the preview)
     - Full real-time edition (can preview menus transitions,...)
     - User can customize the editor
  b) Bad :
     - Intrusive in the application code
     - Force the user to port application on a desktop OS (Linux, 
Mac or Windows), not friendly if he target only embedded devices 
(can be bypassed with a remote system)
     - Less stable editor?

The second solution is commonly used in the video game industry, 
but is the best choice for a larger usage?

What do you think about?