Hello,
2.0 is in the alpha stage, this means that some functions might change their name or in the arguments they accept. No more API changes should happen once reaching the beta stage, but some work is still needed for that to happen (check
this page in the Processing wiki for a comprehensive description of the changes taking place between 1.5 and 2.0). Also, many
external libraries might not work until the authors update them to be compatible with the new version of Processing. In addition, lots of bugs to fix (and find) right now...
However, the ongoing API changes are mostly restricted to advanced functionality. The majority of the drawing functions in 2.0 work at this point in the same way as they do in 1.5. I'd say that for basic use, both versions are interchangeable and sketches written in one should work in the other (most of the time).
For more advanced sketches, and specially if you need to rely on libraries, I'd stay with 1.5 a while longer while the current issues in Processing 2.0 alpha are straightened out and external libraries updated to work with it.
Andres