sometimes, you can only use ' be able to'. such as: 1. Kids might not be able to discriminate between good and bad. [so when there is a 'might, maybe' in a sentence, it's better to use 'be able to' rathert han 'can'] 2. I'm sorry to inform you that our department will be unable to provide service this afternoon due to a power outage. [here is a 'will', you can't say: will can't... ] For certain reasons, I'll be unable to attend the meeting. You will be able to come, won't you? 3.You need to be able to work as part of a team. [here is a 'need to'] and so on...

Nothing. They're the same.