In general, rating is rating. Warmth is mainly a function of loft. And loft can be achieved by down or a synthetic material. Yes, it's not entirely that simple and the minutiae have been discussed in other threads but that's the gist of it.

So....if you want only more warmth and nothing else matters....go for the synthetic.

And, of course, that's assuming that the temp. ratings of those bags are reasonably accurate.

I wrote this before in another thread but I think it bears repeating:

Light.

Warm.

Cheap.

When it comes to sleeping bags, pick any two. You can't have all three.