I suppose that's true, the double bed would depend on your size. Me, I'm definitely NOT 5' tall, and I'm quite Pooh-sized, so a double is much too small for both me and my DH (who's no small man, either).
I guess I'd also be concerned with elderly parents with back problems fitting in a bunk with any comfort, as another poster suggested. Again, only you know what your family can tolerate. Same with sofa-sleeping.