The 944 Demo is a speciality machine set up for use of the demolition arm, and comes with a backhoe attachment as well ('to reduce downtime when there are no demolition tasks'
). It has a bigger and heavier undercarriage for more stability while working with the demolition attachment, which naturally is a plus when only using the backhoe equipment too. I guess this machine is more of an 'german spesiality' when it comes to RC models and not that many are sold compared to the standard excavators.
The 944 basic has a undercarriage matching it's backhoe attachment, and will 'do the job') If on wish a larger capacity machine, then there are the 956/960's. They had up to recent years only the 944 in the product line, which is a 'small' model. A demand grew for larger machines, so the 956/960 was developed (The 956/960 I would guess are about the size of the rc4wd xl4200, without any further comparison, while the 944 is a smaller machine)