I think it's a bit too soon to post a topic like this, while Scirra is still revealing new and new C3 features. But just to mention some of the features that makes it worth to consider C3:
- Scirra's own cloud APK exporter.
- New features in the editor to make the development process faster and more comfortable.
- Completely redesignable and translatable editor.
- New plugin manager with the future promise of editor plugins.
- Built in web-font support.
- Etc..
The browser based editor can have several advantages, and I don't think that Scirra would go for it if they weren't absolutely sure about that it will work great.
The subscription is one of the main topics in the C3 forum section, but to cut it short: No one-off payments. And I think it's better that way (but that's just my opinion).