They are both really good schools but quite different in their approach as far as I can see. I also think you'd be wise to go and have a look at them. It really depends on what you're looking for in a school. Getting an outstanding Ofsted report is quite a technical thing and I think, as a BP parent, it's an outstanding school even if not by the official measures. I also know many people who are very happy with Earlsfield. I'm only going to talk about BP as I don't know enough about the Earlsfield experience to compare the two, but I'm sure someone else will!
Beatrix Potter is focused on the children as individuals. Any formal assessment mine have undergone has been slid into the school routine in the least stressful, most enjoyable way possible. I would say it's almost a point of pride for the school not to get agitated about assessments, and it's certainly easier on the children. They cover everything they need to, but there also seems to be a commitment to extra-curricular learning that I find very impressive. Sport is pretty good for a state school and after-school clubs offer loads of opportunities for sport, drama and languages. A lot of the kids go on to private schools or excellent state schools. The standard of teaching is high, but it's designed to make learning a positive and pleasurable experience.
The facilities are excellent and the PTA is very active so there are decent funds for extras. The school is a lovely environment for learning and playing, and the children have a very strong sense of being part of, and responsible for, the school. The catchment area is quite small so there's a great community and parents are encouraged to be involved both directly with the school and socially outside school hours.
BP is a school with a very distinct character and I think you'll know if it's right for you and your child when you visit. It's probably not perfect but I just asked one of mine what they liked about BP as a school. The answer was, 'Everything.'