Quoted:
I have a Tarptent Rainbow as well. Awesome little tents, aren't they?
Yeah, my first one was a Squall. I still use it if the wife or dog comes along with me.
I love both of them. The rainbow is easier to sit up, get dressed, move around in (and get in and out of), and is a great winter/snow tent, but the squall is a better rain tent.
Being a single wall, and just the way it's made, you really can't get a super taught pitch on it (the squall). Even if you do, it will get a sag going on after a while. The reality of this is that it that it prevents a lot of "blow through" from big raindrops because it has some give to it. The rainbow will pitch tight as a drum head. In a big hard driven rain, it will get some misting.
Just things I've learned over the years from using them both. FWIW.