1. Prof. David Pozar published a paper some years comparing those various tools:IE3D(Zeland),HFSS,Momentum, ensemble,etc.
He wrote that results were quite close, Momentum and IE3D were very accurate (but IE3D was much faster).
I designed a filter not long ago , and there were differences between Momentum, MWO and HFSS. we decided to follow the HFSS results for the production prototype.
Now the biggest difference was in the Return Loss parameter, which for filters is the most sensitive parameter in pass-band. There were differences in the rejection also , so we decided to implement HFSS version. But we still don't have results.
An collegue of mine compared MMIC passive simple components(caps,inductors,etc) and he claimed that Sonnet, MWO,Momentum, HFSS gave for simple cases very similar results compared to mesurements. so he is convinced that they all are the same.
Well for precise work you have to use the correct tool, and as we all know for certain work they should perform the same, but for other work each method has its advantages and cons.