How much do you have invested in Nikon? If you wait a bit you will likely find the rumoured full-frame Nikon mirrorless will be released. If you have no reason to update soon, hang around for a bit and see if it eventuates:

http://www.thephoblographer.com/2014.../#.VRPesOHQAnI

But if you do want to go to the Oly, the Em-1 would be a good choice. Olympus RAW files have always been quite visually appealing straight out of camera, their RAW processing engine must be optimised to bring out the best in their sensors.