Both are great, you are very lucky
Is it for a boy or girl?
Newton prep does go to 13+, but that's usually more useful for boys...
lots of girls prep start at 11, so even at newton prep lots of the girls move on at 11.
For boys, the prep schools usually advise you to apply for 13+ entry if you are at a decent preprep.
Distance is obviously a consideration.
And finally, I'd say go with your gut feel.
Newton is bigger and there is lots going on (which we like, my kids go there)... not only in terms of clubs but also events, competitions, trips, speakers etc etc.
HH is smaller and feels more nurturing (although I have no direct experience, just a feel).
But I genuinely think both are great..