I guess it depends on how well you want things to run - so you need to gauge what your actual power needs are. The Probooks are fairly solid machines, but the Elitebooks are my favourite all-purpose workhorses.
Not sure how much refurbished prices differ but when buying new, Dell's Precisions come in at significantly below HP's comparable-spec notebooks, and they also have a more consumer-friendly design - but I think you get what you pay for. You may want to look at refurb M4700 perhaps.