Know nothing about StCats apart from the very good grades they produce, so ill leave them to others who gp there.
I do know a couple of Tormead Sixth Form parents and subsequently, their girls, and I am a parent myself. The girls there are indeed a mix of very academic and just academic, there are grade boundaries to get in and this year the new deputy head academic has stated a whole host of new inititatives testing regimes in Year 11 and lower sixth to help raise academic attainment, all of which has been very popular. I don't agree with a PP that the clever girls leave, although traditionally half the cohort would go to either a sixth college or co-ed school, usually for that reason, 'co-ed'. Last year however the school had its biggest group stay from Yr 11 into lower sixth and around 5 joined externally. They're very used to integrating the external girls and knowing a number of Tormead girls, they're on the whole grounded, welcoming and happy - therefore im sure she won't have any trouble fitting in. Given the uni destinations and grades they get, your daughter will do well and be helped to do well if that's her academic level.
Agree the school is much more cramp than StCat given Tormead's location, so that's a factor you must consider too. The on-site stcats sport facilities will also be better if she is into sport. But the freedom of being able to pop into town and commute by train etc. helps make that transition from school to uni/work. We've saw the new Sixth Form Centre during a parents event and we're very impressed - I don't think it was done to attract more girls, and the head billed it to us as simply an investment in the girls educational experience - the only centre was very outdated and needed it! The girls do lots with the RGS (including the RGS prep), recently the boys came to Tormead for debating and a formal dinner, and also the Tormead winter formal.
Anyway, I wish you luck with you're choice, sounds like your daughter will do well in any setting, whether it's StCats or Tormead, or indeed at her current school! You're spoilt for good choices in Guildford, which makes deciding all that more difficult! :-)