Full Frame vs Micro 4/3 Revisited with Pro Olympus Lens, article/image comparison.
You have hear me, and maybe others, sing the praises of Olympus' compact PRO zomm lenses. I am sure that many have thought: "Well, I'm sure they are very niiiiice, for the format. But it's only half the frame [and weight] linearly of Full Frame cameras. Surely to get real professional image quality, you need a 'real' professional camera and lens, like Nikon or Canon full-frame..."
This would be natural. In film days, image quality was directly tied to the size of the negative, because a film was a film, and you could not get them sharper than they could make films. But now it's years since the resolution of a milimeter of film was surpassed by a milimeter of digital sensor, so cameras and lenses can be made much smaller/lighter without downsides, excepting low-light performance (and if you need very shallow depth of field).
Well, the test linked to above was made with a current Canon full-frame camera and a Canon professional "L" type lens. And lo: not only did the Olympus lens compare well to it, it beat it handily.
So, if you need to make hand-held photos at night (I'm being literal), go for full frame. If that's not a pressing issue, pick a good camera in a size which is comfortable for you.