Go with Giro. I have a narrowish foot (narrow end of B) and a narrower heel with a wider forefoot. My Specialized shoes are definitely too wide in the heel. We only just got Giro shoes, so I haven't tried them yet. The Scattantes were too wide in the heel even though they fit everywhere else, and same with LG. I tried men's Mavics (did you try Mavic?) and they were close, so the women's would be perfect for me.I haven't tried Sidi yet, but they last forever, so they may end up being cheaper in the long run. And if they fit well, then there's your answer.
The M520 is a mountain pedal, so you'll have to make sure that your shoe is either a mountain bike shoe (fit's generally close between lines, since I think they use the same last), or is drilled to accept 2- and 3-bolt cleats. I have M540s (a grade up; the 530 has the platform around it). Aside from being lighter, I don't think there's a ton of difference. I really like them, but my knees don't seem to. However, I have M545s on my cross bike and I think it's less the pedal and more a gearing problem.
I do recommend them (520s, since we don't carry the 540s) or the 530s to beginners because they're double-sided, so you don't have to hunt around to clip in. They're also fairly durable. The downside with the 540s is that I get hot spots around mile 30, but I think stiff shoes and a proper pedal stroke take care of that to some extent.