Hi,
You can use both all the time. Driven Terminal is easier to set up for all conditions, so unless you really want modal s-parameter data, I suggest you use driven terminal. Driven Modal for waveguides may give better results for some cases.
The difference may occur in calculating impedance. For the case of a microstrip, Driven Terminal can give better results because the H x dL integral for the loop to find current is done differently for the two cases. For the driven terminal case, HFSS finds the conductors and creates the loop around those conductors. For Driven Modal the loop is around where the wave port is, which may introduce inaccuracy because the H field is only strong around the strip and not everywhere on the port.
Joline.