You can't predict, but I can tell you what our experience has been. [Nonprofit genetic research lab with a mix of about 900 Wintel, 600 Mac, 50 Linux desktops.]
Dell is abysmal. We had a new rep about every 6 weeks, and couldn't get a call back from their regional manager--though we'd spent almost $700k with them that year. We kicked them out. Then, we tried MPC. Don't get me started... horrible quality notebooks with about 80% failure rate. Desktops OK, but...
IBM/Lenovo products and service have been far, far better.
You might want to consider setting up with a dedicated rep at a company like GreenPages, CDW, Insight or the like. CDW can probably also offer you your own purchasing site. Remember that service, in the long run, is much more important than anything else, so screen your vendor carefully.