Wow, they are sooooo different and serve two completely different purposes, and look so different. I have both and use both a LOT. I actually use my Courier more, but that's because the Twiggy has more competition from my bazillion Cities, Boxes and Works.
If you need to carry a lot of thing and want a bag you can rough up and throw around, get the Courier. If you're looking for an everyday bag with more structure and a more managable size, definitely the Twiggy is more suitable.