You might search this and other forums for this topic, as I know that it's been talked through extensively before (don't recall if here, though).

Opinions vary on temp ratings, and preferences vary on just using the things.

I don't know that it really helps you much to say that I prefer to not use a liner. More helpful, however, might be that everyone I know with any experience is confident that X ounces of extra down in your sleeping bag is a lot better than the equivalent weight of sleeping bag liner for keeping you warm.

I've used liners when I had to sleep in indoor situations (European huts) that required such, and I used one for quite a bit when I borrowed my wife's bag, just to keep it nice.

In the situation that you describe if it's anything close to weight neutral between those choices, I'd go with the 20 degree bag.