id go with Haswell-E since it uses DDR4. Now I do not state DD4 as it being a huge leap from DDR3 as I would imagine its not, But it is likely the next step so may as well be ready on that front.
The first question to ask is what stats do you have now? How obsolete are you and is there anything that can be recycled or are you wanting to go whole hog and replace everything.
And of course rather than picking what end and how top, id set a budget.