I've tried to find ways of doing full compositions on-the-fly with the RC-20, and it just wasn't happening, so ...
I looked at the Boomerang and decided that the A-B thing didn't go far enough for what I need to do, the sound quality might be iffy, there's no undo, you can't store loops, etc. Some people are using several RC-20s together, or an RC-20 and a Line 6 DL4 (I think), or a few DL4's. I looked into those options, too, and decided they weren't for me. Like you, I need to be able to create different parts (verses, choruses, bridges and whatnot) and string them together, move back and forth easily between them, build whole songs out of loops and store them. It looks like the less-costly loopers can't really handle all these tasks in combination, on-the-fly, but I could be wrong.
The Electrix Repeater (around $800 + on ebay, pretty rare), the EDP ($800 + new, if you can get one--Gibson has problems), and the Jam Man (also expensive and hard to find) are the major options for that kind of flexibility. Joseph Arthur uses a Jam Man for whole songs with fairly traditional arrangements.
Maybe someone else can point out another workable solution?