If Mobi support is important to you, I have to say that the MobiPocket Reader is poor on the iLiad, and (from the reports of owners) very poor on the BeBook (lacking, for example, dictionary lookup and the ability to load your own fonts). Of the devices you mentioned in your post, the CyBook Gen3 has by far the best MobiPocket support. I used to own an iLiad, but sold it and bought a Gen3 because of the horrendous shortcoming's in the iLiad's Mobi reader, together with its long start-up time and poor battery life.
If, however, PDF support is important to you, the iLiad is the clear winner. I'd say really it depends what your main priority is. If it's reading Mobi fiction, get the Gen3; if it's PDF, get the iLiad.