air or vacuum for box in HFSS

Should the airbox material in HFSS be chosen as air or vacuum, which one will give the most accurate result compared to the actual(measured) response?

You can either:
- model the medium present in your real hardware
- realize that the difference is most likely meaningless

Medium would be open air...S21/S11 values changed while using air and vacuum during simulation, hence the question!

But air Er is only larger by 0.5e-3 than vacuum. If correctly modelled, it shouldn't make no significant difference.