Like you say, very small differences on sensor between the cameras.
The main differences come in with features like the 40mp 8 shot picture, better focusing in the em1,waterproofing, image stabilising (whether 3 or 5 axis), veiwfinder res, rear screen res, whether it tilts or swings out are the major things off the top of my head.
Fundamentally all three will probably take exactly the same pic with the same lens and a better lens will make much more of a difference than the body.
Just depends on which features are important to you.
I was all set to buy a em10 mark 2 as didnt need any of the features the 5 offered but managed to buy one cheap on ebay so ended up with the mark 2 5.
From reviews the mark 2 of the 5 or 10 seem to be much better bodies than the older versions and offer better features and slightly improved sensors,