Hi Krystl, hope you like it here in AT
I do NOT have a Kobo (but was very interested at the time it came out and read up on it). I do have a Sony and have to say, I really, really like it and find, that it meets all of your criteria, so let's see...
-- both buttons and touchscreen (solely for note-taking)
Sony has buttons, Kobo is touch only.
-- stable OS
Sony had glitches, which got fixed recently and it's stable now. Kobo has a bit of a reputation for trying very hard to fix bugs and with every firmware update one bug gets solved and a new one appears. I have read, though, that it got quite stable lately.
-- simple loading and functioning (fairly intuitive)
The kobo seems to have a very nice user interface (judging from youtube videos here), but I find the Sony UI also very ok. For library management I use calibre. Simply the best.
-- readablility (minimal flashing, fluid scrolling)
I am not disturbed by the Sony's flashing, but this is very subjective. There are readers, where you can adjust the number of pageturns until the next full refresh. The T1 does not offer this possibilty. While I don't need this (had it on another reader, didn't like it), this is a point of a lot of discussion; ultimately, everyone has to find out for him-/herself.
-- collection capabilities via metadata (i.e. over Calibre)
collections work beautifully with calibre/T1.
At the time, when I was interested in the kobo, collections did not work, which was one of the dealbreakers for me. This may have changed, I don't know.
-- export capabilities of notes
I don't know, but searching the the Sony forums here ("export notes") shows some promising results.
-- expandable memory
yes and yes (micro SD card)
-- "unlocked" for all sources (non proprietary)
not sure, what you mean (read below)
-- landscape mode for event. text pdfs
yes on Sony, don't know about the kobo.
About sources/DRM: personally, I hate being bothered by any kind of DRM scheme. I also witnessed already several ebook-shops going down. Had I not removed the DRM in time, my payed for content would have been lost. So I just buy, wherever I like and strip off the DRM (takes about 2 seconds, it's that easy meanwhile). Ironically I mostly buy Amazon (reading almost exclusively English books).
We are not allowed to give detailed information about this, but google for "Apprentice Alf" and you will get to the right place.
Umlaute work just fine on the T1 (also on my laptop, maybe your OS is not up to it?).
The T1 is on display in shops here, I saw it at Saturn, so maybe you can check it out there.