The pricing for either one is entirely negotiable. I've worked with both companies from the purchasing side, and have seen deals negotiated well below list. Tell either one you're looking at the other, and you're almost guaranteed a 5-10% price drop.
For the end-user onsite, Onesite is easier to learn and harder to break. Voyager really excels at robust accounting, ability to customize, and ability to support internally.
In the end, though... when including continued training, implementation and support costs, Onesite comes out cheaper for just the core product. Add on accounting and other modules, and it evens out a little bit.