I'm sure the vendors would each tell you their simulator is far superior than the other ... and they are definitely different.

I think your question is really: are there significant differences in the implementations of standard models (BSIM4, etc.) between the different simulators.  Obviously, if you use a MOS model that is only supported in one simulator (eg, HSpice's level=66 HV MOS -- I'm sure there are also some in SmartSpice, but I'm not as familiar with it), then this won't be portable.  Also, if you use some non-standard parameters -- the vendors are allowed to add some extra parameters and still call their model "BSIM4" -- then this won't work.  So, be sure to use only parameters in the official documentation for your model.

The Compact Model Coalition has a set of QA tests for the standard models, which are available to its members.