Your question has been pretty well answered by previous contributors but just thought I'd add my comments.
My oldest DS is in year 8 at AGSB which he passed comfortably, along with Sale and St Ambrose. He attended a state junior school that doesn't do any prep for the exams at all and I prepared him for the most part at home using books and materials from this website. I have no teaching experience but found I could guide him through most of it, but would echo what others have said about the Maths paper. The exam includes material that they wouldn't normally have covered at school so if you DIY you will be trying to teach them quite challenging Maths yourself from scratch. Not only that, but they must master techniques that allow them to answer these Maths problems quickly. That I think is the biggest thing to think about and is very much dependant on your own skills, your child's abilities and what your relationship is like. Just before the summer I got a few weeks help for him on the Maths and found he picked it up quickly and actually, what he learnt stood him in good stead for starting school.
Having said all that, i now have the job of getting my youngest ready for the exam in September and I am having him tutored simply because he would not respond to me teaching him at all
I have to be very much hands off with this one but he is happy to see the tutor once a week and do the little bit of homework each week without any fuss. So that's where personality types can also help point to a particular direction.
One other small point which I think has been mentioned is that tutors vary enormously in quality and again personality fit is important, so even if you go down that route make sure you a well informed about how they seem to be doing and particularly how they are feeling about the whole process. If you DIY you will have a very good picture of their strengths and weaknesses anyway. I would also recommended some kind of mock exam just to give them practice and experience of the process, the main point of the prep is to give them confidence and try to alleviate any anxieties they have, it's a big thing for them. Best of luck whatever you decide.