I know its not either one, but we just got an IBM x3650 at work, and its very nice, has a flip down led panel that tells the status of each component so if something goes wrong you dont have to open it up and test everything.
The larger and more critical the server, the more I would lean towards HP. Dell has made several attempts to break into the server market these past few years but they're still not at the level of HP in this area.