Well, I'll go ahead and give you my experience. :D
Last time I was in London in '06 I stayed at the St. Christophers Village for about 3 days and the Astor Quest for about 4 days. Both hostels were great in their own way.
The Astor, i thought, had a great location, almost across the street from Hyde park. I thought it was a more kick back chill hostel if anything. But the whole time their I felt very safe.
St. Christophers was a lot of fun.
It's a really big hostel and it has an attached bar (Belushi's). Even though it was big and crazy, I still felt like the security was good, key cards to get into everywhere. Both of em were pretty close to tube stations as well.
Either way I think they were both great hostels.
Good luck in London, it's an awesome city.