For the Macbook Pros I'd go with a new report since they're using a different core architecture and are adding the multitouch pad.
For the regular Macbooks I'd just update the existing report since the only change is more RAM by default, a higher clocked processor by default, and more hard drive space by default. The regular Macbooks are using the same architecture and form factor it seems.
In the case of the MBPs I recommend renaming the current MBP report to include version/generation information. And have the NEW report be called just "Apple Macbook Pro". That way the "Apple Macbook Pro" report always refers to the current version that is in production, and we maintain the archived info on previous generations that is easily accessible.