I feel there is sometimes a difference. I would tend to only use one of them in:
I can't make personal phone calls during work hours. [note also, no 'my']
The office reception is always attended during/in working hours.
The slight distinction I think I'm making is that 'work hours' are mine, 'working hours' are the office's. But of course this is very tenuous, and there will be much overlap.
My work/working hours are 9.30 to 6. [these are both mine and my employer's]
My work hours are from ten till lunch. [a novelist who sets their own schedule]