NEX got the luscious body. - camera body + APS-C sensor
m4/3 got the wicked tools. - small/fast lenses
So in the end while the NEX goes higher ISO, the m4/3 does not need to go as high due to the increased DOF for similar FOV and faster lenses.
The high ISO performance issue is not as clear cut as it seems.....
At low ISO, the Olympus EP series have always been known for their low AA filter for very sharp o/p and punchy JPEGs.
IQ is the match of any APS-C sized camera at low ISO.
EP3 - Digital Cameras, Olympus PEN E-P3 Digital Camera Test Image
NEX5n - Digital Cameras, Sony Alpha NEX-5N Digital Camera Test Image
NEX and APS-C cameras generally do better ISO1600 and above though.
You can look at more comparisons here:
Imaging Resource "Comparometer" ™ Digital Camera Image Comparison Page