Hi Natsume,
My DS has also had an offer from Radley and he/we have chosen Harrow.
For DS, he wanted somewhere that would support his cricketing ambitions. It didn't help that on the Radley Open Day, Harrow thrashed Radley at cricket!
Academically, I am uncertain if there is any material difference, but the alumnus connection with Old Harrovians appears to be quite strong. I have not heard the same for Radley. Again, entirely subjective on our part and of no interest to DS.
In terms of race, I saw plenty of diversity among mingling of groups at Harrow. For Radley, we were approached by asian students, unprompted, saying how welcoming Radley are and how supported they feel. It left us as parents very confident about the pastoral element at Radley.
On balance, the HM and support Harrow gave throughout the process, including cricket tryouts, being a centre for ISEB (DS prep is not one) and private tours made it an easy choice for DS.
As mentioned before, DS is waitlisted at Eton, but despite our encouragement, wants to stay with Harrow.
Hope that helps.