I've just been on tours of both. Liked different aspects of each. The Thornhill HT seemed much more focused on teaching and learning, food looked good, but the school's quite shabby and the EYFS outdoor space is tiny. With funding all dried up, it seems likely it would remain shabby. William Tyndale has had a brand new makeover and has apparently just got a grant to expand their science curriculum. I also prefer the no uniforms thing, not that it's the most important thing. According to Mumsnet threads I've already trawled, WT is 'arty'. Didn't really see that going around, at least not any more so than Thornhill which has artists-in-residence and seems committed to using drama as a teaching tool.
Would love to hear comments from parents with kids in either of these schools -- what you like, what could be improved, how you think it's different from the other, etc..