it's difficult to say, but Kaspersky has more features and more protection from zero day threats.
kaspersky protect very well but is extremely heavy in "some systems"- it depends.
Qihoo 360 is pretty good, has bitdefender and avira engines, recognized by the excellent signatures, and 360 cloud, to not speak their good registry protection.
on the other hand it has many false positives.
I think that both are capable to detect and prevent new and unknown threats, kaspersky are much more polished that Qihoo.
I use Qihoo and has significants false positives