As a driving instructor I should be saying it is always best to learn with an instructor but I will actually be honest and look at the good points of both!
Firstly the plus points for going with an instructor, and top of the list has to be safety! Our vehicles being fitted with dual-controls means they learn in a lot safer environment, but we also know the latest rules and best way to explain how to do things as easily as possible, with the emphasise on safety!
Compare that to learning with a family member and the big plus point here is you are usually able to spend a lot more time behind the wheel getting lots of practice in before taking the practical test!
In conclusion the idea thing would be a mixture of both but I will add a warning before you rush off to get the car insured to practice in. The insurance is usually quite high so is only worth doing if you actually find lots of time to go out together, all too often I hear that dad is too busy or not off at the same time so that money would be better spent on a lot more lessons with the instructor!