I happen to have both Reactor and SOL bivy. I've used the bivy once over my bag and pad during some unexpected snow/rain. It worked really well to add a few degrees of warmth for that night. However, I did experience some loss of loft in my bag by morning, so I wouldn't want to use it for more than a day or two at a time.
However, the old Reactor I have was billed as adding 15 degrees of warmth, but on a separate occasion when I used it, I wouldn't say that it added that much. The one I have is essentially a very loosely knit fabric. It also wound around me during the night and stuck to my clothes, which was kind of frustrating.
If I had to pick the two, I'd go with the bivy. But you might see if you can slip it inside the bag --after reading up on vapor barrier liners.
But better yet, if you have some time to research, try to find a light and simple bivy that will breathe, since it sounds like you're after something that blocks drafts.