I don't have any experience with the multi-dome but I have a 5' Octodome and I really like it! In retrospect, I wish I had purchased the 7' version.
I have used the 7' Octabox ... but when it came time to buy, I bought the 5' Photoflex Octabox. Why? Because I shoot in my house and there would be NO WAY to fit it into any of my rooms. Also, I brought the 7' octabox on a location shoot and it really limited where we could shoot (due to it's size). Since buying the 5' octabox, I'v had no trouble using it anywhere. It was a little "stiff" and hard to assemble when I first got it, but once I got the hand of how to get the rods into the slots, it was fairly easy to set up.
Here are a few shots using the 5' Photoflex Octabox as the primary light source. Do you see any bad light falloff?