WOW, this is a tough one. Overall, I was always a Carly fan in terms of style. I just never bought one till recently, and I bought a large leather at the outlet. I LOVE it, and I have no problems with the strap slipping (even though I have narrow shoulders). I really don't like the medium size at all, though, and that includes the Zoe.
I've really become a fan of Zoe, though! I think I prefer the way this bag slouches and lays on the shoulder. I think it looks a bit nicer overall, and I think it's a friendly style for everyone! However, that said, I do not like the signature versions of this bag AT ALL... The Carly sig I actually like, but for some reason I can't stand the sig Zoes. I'm not sure why, but that's my own personal taste.
So, if I were buying leather, right now the chocolate Zoe really has my attention. I LOVE the leather on that bag, and it looks fab on. I also like the bronze, but I have seen a display bag with wear issues where it was sitting on the shelf, so I know that smooth metallic is not going to last long.
For sig, I would definitely purchase a Carly, I almost did get the turq. trimmed sig bag at the outlet last time, but I always end up purchasing leather.
As for getting stuff in and out, I'm not sure I notice much of a difference between the bags. In general, I admit I find zippered hobos annoying, and "black holes" at times. But I am comfortable with both the large Zoe and Carly. It's a really tough decision, but side by side, I would probably choose a leather Zoe at this point..just because it's new and I don't have one yet!