If you can build the above system for around the same or less than the HP one - get that one.
- Asus motherboards are the most reliable IMHO. Not the best for overclocking, but I doubt that you're going to want to be OCing on a video editing system anyway.
- 2 hard drives in the above system. 2 HDDs are best for video editing. Plus the one video drive i the system above is bigger anyway.
- The graphics card is a higher spec (not that this really matters if all you're using it for is editing)
Also bear in mind the HP one doesn't come with a monitor or a capture card (especially one of canopus quality)
Are you building it yourself or getting someone to build it for you?