It's hard to make a true comparison without knowing the exact jacket, pants and sleeping bag you want to compare.

But in general the bag will be warmer because it offers less surface area to lose heat from. There also could be a gap between the jacket and the pants that will lose heat. You can hunker down in a mummy bag, close up the face opening, and stay amazingly warm. Can't do that with jacket and pants. Yeah, you can close up the bivy but it's not insulated. It would help if the jacket had a hood but it's just not the same as a bag.

Are you considering this to save weight? Or for versatility? Or....????
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If you only travel on sunny days you will never reach your destination.*

* May not apply at certain latitudes in Canada and elsewhere.