Is there any special reason you've dismissed the Sony PRS-T1 already?
You won't be restricted to the Sony store (neither will you be to the Kobo/Kindle store should you buy one of those). Sony and Kobo both read Adobe DRM protected e-pubs and everything that hasn't got DRM is easily converted (not PDF though).
Since dictionaries seem to be important to you I'd take a closer look at the Sonys. You haven't said where you live or what your preferred reading language is, but the T1 (I think the dictionaries are the same on the x50's) offers both the Oxford English (UK) and New Oxford (AE) as well as English <-> German/French/Spanish/Italian/Netherlands dictionaries.