Both machines are built very well. The Dillon has more pieces, many of them small and plastic to do the same job as the Hornady.
Both have the same warranty but Dillon REALLY advertises theirs. Hornady offers help with load data as well over the phone.
The Hornady is priced less. Caliber changes will be cheeper with the Hornady.

Hornady has a better powder measure.

The machines do differ some as far as where the brass is loaded, to me the Dillon felt "left handed" so I bought the Hornady. I sugest you try both before buying.

Here is some reading on the subject.
http://www.cs.odu.edu/~rtompkin/hornady/blue.php
The last paragraph sums things up nicely.