I'll preface this by saying the usual and necessary "it's too early to tell, who knows what might change, etc." Obviously it depends upon the acquisitions we make and the like, but for now I'd be inclined to go with Harper. As the rotation stands RIGHT NOW, Strasburg probably isn't AS necessary as a good hitter RIGHT NOW. Furthermore, I think Harper has the bombast needed to revitalize DC as a baseball city. Having a career guy with attitude like Harper gives the Nationals a rich history with just that one player. I think that on the field their impacts may equal/cancel, but off the field Harper will give the Nationals an identity for years.
Of course, all that could change the second Strasburg throws a perfect game, or with one FA acquisition. But that's my preliminary thought.