The lunar duo is huge for one. It's 90 inches long, and pretty much that entire length is usable when you are lying down. So if you are 78 inches long, that still leaves an extra foot for your sleeping bag head/foot and to stretch your toes out.
The double rainbow is 96 inches long when flat, and 88 inches long when the bathtub walls are up. So it's just 2 inches shorter than the lunar duo in bathtube mode… still plenty big enough.
I don't see why either would be a problem for you.
Which to choose would depend more on whether you are going to be sharing with someone else, and how big you are in girth. The lunar duo has quite a bit more usable space in the width direction, so if there are two of you and you're a big guy (wide) then I'd go for the lunar duo.
If you're considering it as a solo shelter, why not choose the 1-person rainbow?
If you use trekking poles the double rainbow (and single) gives you freestanding setup (of sorts). It also allows you support the main cross struts with the poles, making it probably more strong wind-worthy.
However, in terms of usable space inside, the lunar duo is the biggest.