This really depends on how much detail you need for the converter. In general, when doing powertrain analysis, you don't need to worry about the control loops on the converers as they are simply so much faster than any of the vehicle dynamics they have no effect on system level performance (assuming you trust your controls engineers to have closed the loop well). In that case, using a behavioral dc/dc converter will work better because you can skip simulating the switching frequency of the converter. You can do this using either PTBS or Simscape blocks. However, if you use simscape, you will want to use the blue components instead of the black components. The blue components use a newer solver technology that is better for implementing behavioral models and has more access to high quality battery and fuel cell models compared to the black components (specialized power systems).
There are free onramps on simspcae for simulating circuits and power electronics as well.
