Mixture
As has been stated earlier, building your own PC can make a difference as you get what YOU want without any added "extras". Also, the features on the motherboards, etc, are a bit "restricted" compared to a self-build or one built ba A.N. Other to your specification. And let's not forget how the power supplies on things like Dells and HPs are on the limit for what is fitted "as built by te company".
Oh, and I have looked inside an HP recently. Stepkids have a Core2Quad one. A decent machine but absolutely NO upgrade possibilities without removing things.