The only difference may be class sizes in the pre-prep, but I think if you look further up there will be more differences.
What is the structure like from Y3 at the school you are looking at? My DS (Y3) now has all his lessons taught by specialist teachers, the kids move round to the teachers and Science is in the labs, DT is in the workshop, ICT in the tech centre, French in a language suite, Art in a beautiful bespoke studio, Drama in the performance space and they use the theatre belonging to the senior school for shows. He also has about 10hrs of sport a week across PE, sports coaching swimming lesson/ squad training and matches (at least one match every week, match teas are a big thing). It is really pretty different to KS2 in the school I teach in.
As you both work full time the wrap around could also be a big deal. My DS can come home every day at our choice of 4.20pm, 5pm, 6pm or 7pm. If he were to come home at 7pm he would have had dinner. He could also book in to sleep over on an ad hoc basis. He hasn't done either of the latter two things yet, but I'm sure he will when he is older. Coming home late is not like being 'left' at school- because it's a boarding school lots of kids are still there, the staff are still there and the school is still busy and buzzing well into the evening. Makes working FT so much easier. He is in school on Saturdays though and has very long holidays, which might be less appealing to you.
Anyway, I recommend you make sure you look further up the school and see whether the set-up will work for you/ your DD.