I'd like to start developing a new project using sproutcore. Since 2.0 seems quite different to 1.6 and there are already three betas out (and so I expect and RC soon?) I wonder if it'd be a good decision to start directly with sproutcore 2.0 intead of 1.6.
The sproutcore app will be backed by a rails app which exposes a rest json api.
Sproutcore 1.x and 2.x are indeed targeting different types of applications. So, the decision to choose 1.x or 2.x mainly boils down to the question which application type you are going to develop.
Choose 1.x if you need a set of predefined components, e.g. if you plan to develop an internal CRUD-like application. You might use the new template-based approach in some places but your main application will be composed with predefined components. SC 1.x clearly targets desktop-like applications.
On the other hand if you plan to build the next twitter or github or stackoverflow, you should use SC 2. It's easier to embed into webpages and you are in control over the complete layout, html and css but it is clearly more work to do in regards to html/css. If you've to implement your own design it's probably easier with SC2 because you are in full control. If you've already profund jQuery knowledge you can use this with SC2, it's no problem to combine the two, in fact since SC2 fully builds upon jQuery it's already included ... where SC 1.x only uses a special stripped down embedded jQuery version. If you plan to use certain plugins this might be a problem.
The programming model for your model and controller parts is nearly the same and it is very easy to transfer those parts from SC 1.x to 2 (and vice versa), the main difference is the view part.