OK, you seem to be missing the point.
1. You can build a machine using the same components for less money.
2. The pretty cases they use do not make the machine run any faster than a plain beige case.
3. The service contract and your own build ability are the only factors that play a part in selection of a machine other than the components.
NO company can build a machine better than any other company, with exception of maybe places like go-l.com which overclocks and tweaks everything to its limits, and you pay a very high premium for those monsters. For the most part, all these companies use the same basic components, which just happen to be the same components you can buy. <sarcasm>I know its a wild concept, but its true</sarcasm>