I think a 5k iMac would be a good option has the performance on par of the Mac Pro, its a fairly recent update, and you get a 27" display.
Since the OP posted this in the MBP forum, there may be some mobilitiy needs, that being the case, then wait until WWDC and see what the new laptops from apple looks like.
Even in the remote chance apple updates the Mac Pro at WWDC, I think its not a choice I recommend, given the configuration, and closed nature. Its easier to justify a 2k expenditure with limited upgradability, then something that will cost over 3k and not be very upgradeable.