I own both the EM1 MK II and the EM1-X, and I would say firstly from an image perspective there's nothing between them. TBH the EM1X is the nicest (and lightest) integrated grip body I've ever used (and I've owned Nikon's D2X, D2Xs, D3, D3s, & D4). Ergonomically it's just about perfect. Obviously as you probably know the Em1X has a few extra bell's and whistles up it's sleeve (HH High Res, Live ND, Specific Subject tracking modes, more IBIS adjustability, more focus options in Video like speed, an AF joystick, built in GPS, a slightly larger view EVF, twin UHS II card slots and more buttons for ISO, WB etc.) With regards to the subject tracking modes, whilst this is currently restricted to Cars, Bikes, Planes and Trains, there is hope that Olympus will release a firmware update to include animal and bird tracking as well, although this is more rumor and speculation. But the EM1X with it's twin processors certainly has more HP under the bonnet.
If however you are just talking about pure image quality and general purpose AF, then TBH as said there's nothing between them. When I use my long lenses for birding and wildlife photography (using the 300mm F4 pro with or without 1.4x TC, or the Leica 100-400), then I always used the grip on my EM1 MKII, so in that case, the EM1X is actually no bigger or heavier but has the advantages mentioned above (especially the AF joystick). However if I want a small and light body for general photography or travelling then between the two it would always be the EM1 MK II.
I think what I'm trying to say is as good as the EM1 MKII is (and it is
very good), the EM1-X by virtue of it being 3 years newer probably has more headroom for firmware updates than the former and as such just feels a little bit faster and snappier. Also, I would speculate that as the EM1 MK II is over 3 years old, that a MKIII version is probably round the corner next year with no doubt it being a "mini" EM1-X. Having said that the EM1 MK II was launched at nearly £2k (I know I purchased mine for £1,850 back in December 2016), and I doubt the MK III will be much if any cheaper at launch. I also note that i you don't mind a grey import, that Panamoz are currently doing the MK II for £940 new (which in my opinion is a steal).
I must have owned 40-50 cameras in my lifetime, (mostly Nikon starting with the F3 film), and guess which one I have owned the longest out of all of them......The Olympus OMD EM1 MK II - I've owned this for nearly 3 years and to be honest will never sell it and will own it until it dies. It has been (touchwood) bullet proof reliable, no peeling grips, no mechanical issues and never had the sensor cleaned (and it's still clean) in all that time. Sorta tells you everything you need to know doesn't it
I hope this helps.