601/604 et al
You are fortunate to have a boss that is actually considering an upgrade in this economy. Just hanging on to what you've got is a victory nowadays!
The 600 series Challengers are great values, but the key feature that sells them is the cabin. It's great! And since the man who writes the check also rides in back, that's what appeals the most.
As such, he may not see much difference between the 601 and the 604, except in the price. However, things have a way of equalizing in used aircraft values, so what may seem like a good deal up front ususally means higher costs later in operating and upgrade costs. Because of that, I like to recommend buying the newest aircraft possible.
The Challengers were never really performance leaders in any catagory, but do have well-rounded performance. But, once again, the cabin steals the show, and the pilots have to deal with whatever performance compromises there are.
I've always been a big fan of the Falcon design philosify, so I would be remiss if I didn't put a plug in for their products. The 2000/2000EX competes very favorably with the Challenger, with an unmatched safety record. That slatted wing makes a world of difference......