I'd say the biggest factor is what you're going to be doing.
When I did my final BIQ field ex in Meaford and in February, I only took my inner - but, IIRC I never got the chance to use it anyways. On that ex, most guys just just racked on their thermal rests with maybe a ranger blanket. No one bothered to strip down and hop in the sack. On the other hand, I spent the past week living in a heated mod tent on a cot (Able Bear). It wasn't nearly as cold as Meaford, but I brought both sleeping bags. Even though I wasn't actually in my outer, I had it in my bivi mag and used it as another layer to sleep on top of.
My best advise: ask the other guys in your platoon. Different units and different trades have different requirements.