Welcome, Gwen ^^
Based on your petitions, I'll give my stance about the devices I know better because I've tried them, the three 5-inchers: Pocketbook, Opus and PRS-300. I chose the Pocketbook, so it's the one I can speak more in depth about.
1) I find the Opus and Pocketbook most compelling design-wise.
2) Thanks to the Calibre program, managing your books shouldn't be a hazard regardless of your tech-savviness and the ebook reader model you choose. However, I dislike very much Sony's official software, after suffering it in a PRS-700, and I like much better Pocketbook's and Opus' stance, which is simply treating the thing like a USB stick.
3) Pocketbook beats them all here.
4) Pocketbook is quite rapid, Opus is a bit behind, and Sony is seemingly a bit on the slow side. However, it's something you get used to, and it shouldn't really influence your buying. Ereaders should be able to stay on for days without significant battery consumption.
5) None of the three have it. I think a touch screen is more useful the bigger the screen is.
6) Pocketbook offers you expandable memory and replaceable battery, if I recall correctly Opus does too, and Sony has both a non-replaceable battery and no memory slots.
7) None of them have it.
8) 5 inchers and magazines won't like each other. As for reading normal texts, the number of grayscales is irrelevant. The screens of the three are very good.
9) My experience with Pocketbook customer support is VERY satisfactory, mainly through this forum. Sony's is quite satisfying too, in their own way. Opus, on the other hand, seems to have an "absent landowner" business model.
My order is Pocketbook 360>Opus>PRS-300.
Oh, and the noise of Pocketbook 360 is kind of a legend here, everyone has watched that video
But the main thing is that the camera's mic is very close to the buttons, so the button click sounds are overestimated. I have seen no owner of a Pocketbook 360 who has complained about it. Plus, mike_bike_kite has pointed you to the famous FBReader180 which allows you to turn pages with the G-sensor and touching no buttons. The feature works better every version, and even if I wasn't very convinced of it at the beginning, I have ended up using it because it's quite nice.