St Catherines is a Catholic school, just for girls, from 3 - 18 TPS is for both boys and girls and has a strong Christian ethos. Girls leave at 11, boys at 13, I believe. So they are quite different.
Depends what you mean by best. Best pastoral care? Best facilities? Best ethos? Best sports teams? Best exam results? Best SEN support. Cheapest fees? Best journey from your front door to theirs? Best aftercare? No one can tell you which one will be better for your child/ren.
Both are well regarded round these parts, as are Newland House, Jack and Jill, Denmead, The Mall, LEH, Hampton.... the list goes on. There really are a plethora of great schools in the area. The state schools are also extremely well regarded at junior level (<11) and for girls, the highly prized Waldegrave senior school. I don't have any knowledge about senior state schools for boys, I'm afraid, so can't comment on those.