Hay 7 is a more upmarket place to live, ya3ni more expensive, generally cleaner and more western style amenities like western restaurants and shopping centers and department stores. Hay 10 is definitely a lively place with many students from all over the world and flats can we found for as little as 600LE (£60) a month for a 2 bedroom flat. The area can get really conjested on the weekends when the ‘car market’ is held, sometimes blocking the main road.
Depending on your budget and the standard of the flat you expect and the ease of accessibility to western style amenities you’ll know where is best for you.
I lived in both and with the money I saved living in Hay 10 I lived in a really nice flat in Hay 7.
And Allah knows best