Neither. 870+P55 will save you a lot of money and still have same/similar overall performance.
Systems with lower BCLK tends to be more stable and easier for OCing. 870 has 22+2 multipliers and therefore you can achieve 4.2GHz with merely 175MHz BCLK which shouldn't cause any instability. However, most i7 (9xx and 8xx) can NOT reach 4.2GHz without turning off the Hyper Threading which is the point of buying an i7. I would recommend you OC it to 3.6GHz~3.8GHz with HT enabled.