As of July 5th 2015 mirrorless is not ready for prime time. But who knows what a year or two or three will bring. I have no intention of ever changing or "upgrading" to mirrorless. My next purchase in a DSLR will be a 1Dx if I buy anything. One good reason to buy the best when you do buy is, it stays viable for a longer time.
Not many, if any, cameras made today top my 1D Mk IV and 1Ds Mk III. Yeah, a few may have a better paper spec here or there but not a real world difference.
If you truly want to upgrade, get into a 1 series. You will never go back to a lesser camera. But you do as you see fit.