The imperfect progressive expresses an action that WAS IN PROGRESS in the past when it was interrupted by another event, was being done at a certain time or when something happened, emphasizing the continuing nature of the act. So "estaba + gerund" is similar to "was + -ing". In Spanish is normally used the progressive form in such cases and actions. We use the imperfect to talk about a sequence of actions that took place one after another. So if you say "comía cuando sonó el móvil", rather you indicate that you stopped or interrupted eating when the mobile rang. In addition to this we can't use in Spanish the present progressive tense to refer to a future event (as in "We are leaving tomorrow" in English). So it's said in Spanish "Saldremos mañana" or "Vamos a salir mañana".

As far as I can see they are the same. I just asked my Mexican friend and he said they were the same too. I think in literary contexts 'comía' or 'hacía' etc would be preferred instead of 'estaba comiendo' or 'estaba hciendo', because they're more precise, or elegant. What is more difficult is that the imperfect (past continuous) is used in contexts where an English speaker would use the preterite, for example, 'en aquel tiempo tenía 5 años' = I was 5 at that time.