I'm in the same situation as you and that's why I initially switched from paper to ereader. First I bought a cybook gen3 over the prs-650 because I could install custom dictionaries (not the kind which only knows what you already know).
But it turned out that the UI was not good enough (no touch screen, and bad ability to detect plurals and so forth).
So I eventually tried the PRS-T1 with its touch screen and embedded dictionaries and I'm really happy about it, both by the dictionary content and accessibility.
NB: I don't use the English to French dictionary but the plain old English one which has much more details. I guess you would do the same if you're used to read in English ?
Hope this helps,