I think a lot of buyers will be looking not just at single core speed, or just multi-core, but will instead be looking for the best mix of the two for their particular work.
For me, I lean towards the higher single-core speed because of my use of Photoshop, but I do some 3D work (though a small percentage of my projects), and for that I need a decent multi-core speed as well. For my work flow, the 6-core might be the most effective as it represents a good mix of high single clock speed and moderately good multi-core speed.
At least that's what I'm seeing in the (admittedly) limited reviews coming out right now.