Congratulations!
I had the same choice and opted for St.George's, having received excellent care there when I had an ectopic pregnancy earlier on the year. I am now sixteen weeks pg and am very pleased with my decision! Having had the ectopic I had to have two early scans, both times I was seen quickly by experienced midwives. Incidentally, in the unlikely event anything should go wrong early on the St. George's EPU is walk-in self-referral between 8am and 11am.
Once I was given the all clear to join the 'normal' care route, I was again impressed by the fact that the midwives and sonographer (at 12 week scan) were again very experienced.
Another attraction of St.George's was the fact I could have my booking in appointment on a Saturday so DH and I didn't have to find excuses to escape work prior to being ready to tell colleagues I was pregnant. My routine care is during the week but in my local health centre, so although it's not a huge hardship to get to the hospital this makes things that bit easier.
I think that both hospitals offer a birth centre delivery. I toured the maternity unit early on to see the facilities at George's. However, I have been told that St.George's is the 'go to' hospital in the unlikely event that anything should go wrong with delivery due to the resident specialists and facilities.
Finally, although I work very close to St.Thomas's I live much closer to St.George's and DH and I felt that if all goes well the baby will arrive once I finished work and being that bit closer for a first baby, when we may well get to the hospital and be sent home etc. would be better. I doubt in reality there is much between the two hospitals so ultimately this was the deciding factor!
Good luck!