I've been reading through many of the 2-stage threads here, and the overwhelming preference seems to be for lighting the sustainer from the sustainer. I can see the advantages:
- Separation charge to get booster clear of sustainer before ignition
- Sustainer still lights even if booster prematurely drag separates
- Sustainer motor potentially roasts the booster
- Sustainer won't light if the booster prematurely separates