I struggled with this choice for the March '08 birth of my first baby as I'd heard bad things about both. I initially chose C&W without going for tour of either. I researched both and checked out birthchoiceUK website and figured it was much of muchness. My only experience of St G's was a miscarriage last year, and I really didn't want to go back there for my labour. Mostly down to emotional associations of that time. It also reminded me of "ER" (the TV show), but with worse lighting.
Ended up going on the tour of C&W and thinking, well, it's a hospital. Not plush or fancy, and the L&D ward kind of grim as most wards are. I was a bit apprehensive, but figured it'd be OK.
But during NCT classes (with most of others going to St G's) I panicked and thought I should just look at St G's new birthing suite just to make sure. The facilities there didn't impress and I decided I'd stick with C&W, figuring the staff are the most important factor and you can't really get a sense of that on a tour.
In the end, the care at C&W was very good. I had an excellent team of midwives (one student/one senior) and surgical team (placental abruption followed by emerg c-s with a spinal b/c no time for epidural). C-S rates are higher at C&W, but out of 6 NCT ladies, 3 were c-s and 2 of those were at St G's. The surgeons at C&W saved my life, so I'm pretty pleased with the results. Baby was in NICU for monitoring for about 24 hrs and the staff there were good, too.
Post natal ward...pretty grim. I was in for 2 nights and couldn't wait to get out. And I was in my own room!!! I've heard that of both hospitals, though, and imagine that if you have a straightforward birth, you'll be hot-footing it out of hospital as quickly as possible. My NCT friends who went to St G's said they liked their L&D midwives but post-natal staff were pretty rubbish.
I really think it's down to the people you get on the day that make the experience. If you are concerned about the care you get, I'd consider a doula. I had one and it made the experience much less scary and I liked the continuity of care. Sorry so long! HTH