Actually, HFSS is a frequency domain full 3D EM analysis tool, based on the Finite Element Method (FEM).
Fidelity is a time domain full 3D EM analysis tool, based on the Finite Difference Time Domain (FDTD) technique.
Each has its relative strengths and weaknesses, so which tool is best really depends on your application and preferences. No one can say that one is better for all applications. Some problems are more efficiently solved in the frequency domain, others are more efficiently solved in the time domain.
--Max