I think having the same instructor all the way through training is a must, mainly for consistancy, but you need to find someone you click with. The majority of my training I had with one instructor, she was very down to earth and wasn't affraid of calling you a ****, if you were flying like one! This suited me as I much prefer to be told I'm flying badly than have an instructor be too nice and worry about offending me!
However, I did find it very helpful to fly with other instructors especially later on in my training as they always pointed out things that I hadn't looked at before, this is in no means a bad reflection on my main instructor, but sometimes when someone explains something differently it makes more sense and in the long run makes you better for it.
Go with who you feel comfortable with but don't be affraid to change if you are struggling.