[iacchi]

Hi! So, I'll give you a partial answer now and the rest of it later on (especially for the TTS, I'll try to make a video as an example) because it will take a bit to test everything and I don't have the time now.

First of all, I got the C67ML in the end because it was cheaper for me and because, as an Android system, I could install useful apps that I couldn't have on the Pocketbook (e.g. TTRSS-Reader, to read my news feeds on the device). Depending on your use, I'm sure both will be able to satisfy your needs.
Coming from an old device with a Vizplex screen, the carta HD screen is just amazing. The contrast is so much better, the resolution of the letters is just perfect (definitely no difference compared to actual paper) and the ghosting is just nonexistant (unless you use the backlight at a very bright level, bu I usually keep it veeery low because it's enough) even if you set a ridiculous amount of full page refresh (I think I've set it to one every 30 or 40 pages). Both devices should have the same screen, so no difference here, just enjoy the greatness!

As I said I'll come back to the TTS later on. Hardware wise, the buttons are placed in decent places for my handling. I either keep the device on the bed and read from above (so button placement doesn't really matter) or I make an L with my thumb and index finger and hold the device from one of the two bottom corners (depending on if I use my right or left hand). In the latter case, my thumb doesn't hit the lower button and my index finger rests on one of the page turn buttons, but the buttons are sturdy enough that I don't click them by mistake, unless I want to (which is also useful because I can turn the page without changing holding position). Despite the device being light in weight, I get tired pretty quickly in this position anyway, so I usually prefer ho have it on the bed.

About the filing system and what to use to load the books. Unless you tell the device not to, it will scan every single folder on your file system to look for books, and add them all to the library. The other setting is to limit the search to the Books folder (both on the system storage and the SD card) and its subfolders. So, you can load the books through Calibre, your file system manager of choice, or whatever you prefer. You can also install dropbox or the like and use that for all the software cares.
One thing, though: you have two ways to access your books (unless you install a different reading software and use the built-in functions of that software, in this case it will vary). The first is to use the library system app, available from the home screen. With this app, it doesn't matter where you place your books, it will display them all in one big list, without nesting.
Your other choice is to use the file system browser, also placed in the home screen. In this case, you'll probably open the Books folder (or whatever other folder you choose), navigate it until you reach the folder with the book that you want and open it. With this method all you'll read is the file name rather than metadatas, but it's still pretty simple to use.
Personally, I just use the normal file system manager of my OS to copy the books inside the reader after having connected it to the USB cable, because my Calibre library is very outdated and a mess at the moment. I will eventually switch back to Calibre, though, and I don't expect any problem.

For reading, I'm using the kindle font (Bookerly) because I like it. In general, if you want a font other than the preinstalled ones, you just need to create a "fonts" folder on the root of your file system and any reading app will find them there.
One thing that the Onyx has and the Pocketbook maybe doesn't (I don't know) it's the A2 screen mode. This is quite useful to e.g. surf the internet or in all cases when you need to scroll up or down. It reduces a lot the quality of what you're seeing (basically, it becomes black and white rather than a gray scale) but the responsiveness when scrolling the page up or down is almost the same as the one of a LCD screen, so pretty good. I use this mode to read my RSS feeds, because I usually need to scroll a single article up and down when it doesn't fit a page (the page buttons will navigate through articles). The text remains very good and readable, just don't expect to understand any image.

The only problem I've found so far (and I don't know if it's just my device that has some problems or if it's a common problem) is that sometimes it doesn't switch on. You push the switch button (by the way, you need to hold it for a few seconds, otherwise it won't switch on. It's a bit annoying when you have to actually switch it on, but it definitely prevents any casual switching on when you have it in your bag) and the blue light appears as it should, but then the screen and system doesn't turn on. In this case, the only thing you can do is to have a needle at your disposal, hit the reset button, the blue light will go away and when you press the switch button again the system will start normally. I should probably send it in while it's on warranty for this problem, but it's so minor that I can't be bothered.