Purchase Decision: OM-D EM-5 or K5II
They are both awesome cameras. But what is going to be used more when you're on the go? I chose the 4/3 because I take it everywhere . I originally bought a PEN-e-PL1 . I bought/tried/returned a canon rebel, panasonic dmc200, a sony A57, and a sony Nex6. I kept the PL1 because I could not find any difference in quality on blown up and cropped pictures. I was floored by the size of the canon and sony dslr's and really didn't want to lug them around.
I was so impressed with the PL1 that I bought the OMD-F EM5. I upgraded the lens to an 14-150mm lens.
Since they all take awesome pics, for me it boiled down to quality, ease of use, carry size, and easy use of menu.
Hope this helps