I don’t think you will see much difference in power consumption. Both CPUs have speed step technology, and the same voltage range. Personally, my first rig used an E3200, and it was cripplingly slow for normal tasks, even with an SSD. Think you would be better off using the E8400 and trying to undervolt it.
Allow me to ECHO what this guy said. Don't concern yourself and waste time on things with very small details that will probably have little or no impact on your long term mining profitability. Instead focus on sourcing your hardware as soon as possible so you can finish building your rig and start mining! Remember that our days are numbered till ETH goes POS.