They are interchangeable. Both of these sentences sound correct :)

both are right, but hard work\job can mean intense not just difficult

I believe it would be best to say: 'I have a difficult job'' (correct) and/or 'My job is hard'. To have a hard job seems, to me, to sound like your job is very dense, like a rock is dense/hard. It is more colloquial to say "my job's tuff/hard" or the less formal ''My job sucks." Either way, the meaning of your idea would be understood so I would not worry about it too much. Nick