I'm currently in the middle of making the same decision. I had my first child at C&W, and had a good experience there. It wasn't an easy birth but I felt well taken care of during the whole process.
I booked into St Thomas's for my second, because it is more convenient for work, but have not been very impressed with the admin so far. I was not offered a booking-in appointment until around six weeks after I self-referred to them, for example. I know this isn't necessarily indicative of the care I would get later on during my pregnancy but I think I will feel more comfortable switching back to C&W.