I think there are pros and cons to each and you need to think about how these apply to you and your daughter.
I was a first timer this year and I've tutored my daughter myself. I am a maths teacher (which doesn't make too much difference in itself at all) pros
I could plan her study timetable myself and use time to reinforce and practice topics etc. that needed going overcons
I could push her more than maybe a tutor would because I know her capability better
more control over and knowledge about what and how much she's learning
my daughter didn't feel nervous/feel the need to impress me when studying with me as she might with a stranger.
she knows me better and how to push my buttons - it's difficult to not show the same amount of exasperation with DCs as you might in every day situations - very counterproductive when studying
personality clashes when workload increased- she's a bit of a delicate flower and I'm not
can be hard to study with younger siblings around
I enrolled my daugter on an intensive 3 morning summer 11+ course which she attended witha friend from school. both girls really enjoyed it and I wanted to use that as an independent source of her ability and progress etc. but also because DH and family were scaremongering and expressed doubt when I said she was doing really well.
She also did some mock tests to practice in a real test environment.
In hindsight (and because I had no information previously) I think the course was at too easy a level for her by that time. the mocks were a very useful experience and gave us a snapshot of current achievement and good feedback.
My advice would be that if you have time - then tutor her yourself. You can post practically any question you can think of to the forum and it will be answered (if it hasn't already - do a search first)
If you go the tutor route then 1 to 1 has to be the way because even if it's two students ( I helped coach some of my daughter's friends at the same time) then one's attitude will rub off on the other - the tutor's attention by default will be divided - student might not feel comfortable raising issues in front of the group etc.
I'm sure someone else will come along and share their views - you must definitely do what feels right for you and your daughter