The Rainshadow has more floor space, since it's a 3-person tent. However, because of the slope to the sides, not all the space is usable. If you really need space for three, then go with the Rainshadow. However, for two adults the Double Rainbow has the advantage of two doors and two vestibules. That's much better than climbing in and out the front door of the Rainshadow, planting your feet in the faces of the other sleepers! The Rainshadow's larger footprint can put it at more of a disadvantage when you're looking for campsites.

I have a Rainshadow (for my 3 grandkids) but not a Double Rainbow. I know if I were going out as half of a couple, though, I'd definitely want the Double Rainbow!

EDIT, later: Do you use trekking poles? If you don't, the DR will be lighter because you'll need to buy two poles for the Rainshadow. (Even with trekking poles, the Rainshadow is far more stable with two of them.)