Marie:
I'm a huge fan of both AG and Magenta. But not a huge fan of the Day. Having said that, I think the Magenta, while also a strong colour, is a bit more versatile. There's something about the AG that is more "fun" and youthful and sort of saucy. The Magenta is a bit more of a "grown-up" colour. I would carry both year-round (they are Bbags after all!), but I would be a bit more comfortable carrying the Magenta in a greater number of situations.
Also, I have to say (with the devotion of the newly converted!!), the Work is a perfect size if, like me, you carry most of your belongings with you everywhere you go. I have heard that the Day can be a bit of a black hole, but I don't have first-hand experince with that.