In rendering the X6 perform about the same as the i5/i7 quads. In general posing, clothroom, hair etc, the i7s are better because the cores are faster and all these things in poser only use one core... So overall I think the Intels are better value.

You will also eventually be able to get cheap Intel hexacores, which are a lot faster then the X6, so you may wanna wait for those instead (but at least 6 months wait.)