As a general thought, I would never buy a device in the hopes that they will one day support the format you want. This is particularly true in an economy that might well cause some of the smaller vendors to go belly up (Not through lack of sales, so far ebooks have resisted the trends in the market, but because of credit issues).
Lets just break it down.
1. You have lots of Mobi files, but you can deDRM them and then convert them to other formats.
2. Many of the new books you want are being offered only in ePub.
3. ePub books should be fairly easy to convert to other formats provided they do not have DRM attached... but naturally a lot will.
4. The Sony supports ePub and your Mobi books can be converted to LRF or ePub fairly easily using Calibre.
5. The cybook will support you mobi books, but not your ePub...
Personally, given your set of criteria, and the fact that the other factors (That I did not include) you listed were mostly personal preferences, I would go with the Sony...
Of course I did none of the above when I bought.. but then again, I have a thing against DRM to the point where I wanted to buy a reader that didn't even support a DRM'd format.
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Bill