There is no inherent difference between these two groups as neither has any permissions assigned by default. The only two groups which matter OOTB are Administrators (which can do everything) and Users (which can do almost nothing, assuming you remove Users from the Everyone organization as is recommended). Designers and Developers are just two good default names for groups to which you may want to design varying amounts of design and runtime permissions.
There is a basic overview of ThingWorx permissions found in KCS Article CS236180
I hope this helps!