Both HFSS and Ansoft Designer can be regarded as accurate... Given identical models, both tools should provide identical results within the numerical precision of their respective computation engines. The challenge is in the care taken by the user to ensure that the models are indeed identical.
Nonetheless for planar circuits, Ansoft Designer will generally reach a solution much more quickly than HFSS. At the conclusion of design development using Designer, many users will verify using HFSS if they are fortunate enough to have both tools...