Spin-offs - during the main show or after?

I'm watching New Who with someone who only ever caught some of the classic show in the '80s, and having never seen the RTD (or Moffat) era spin-offs I want to get an idea of what other people think. Is it best to (more or less) watch Torchwood/SJA/Class in between bits of DW in the way that it was aired, or watch them separately after catching up on the main show?