WARNING - This post contains opinions of an impulsive nature.
I've uploaded a couple of high-res files to Google Drive which you can access here. They are comparisons between similar images shot on a Nikon D600 with its "kit" lens the 24-85mm and an Olympus OM-D E-M5 with its "kit" (well one of them) lens the 14-42mm. I'm not going to say too much about this as its for you to make up your own mind as to which you prefer and which you would find more useful.
The files were all processed identically and to take account of the fact that the increased number of pixels in the D600 is an important factor in peoples decision to go with that camera, I've upsized the m4/3 file to the same size as the Nikon.
A few brief observations.
Despite the upsizing and much-maligned kit lens the Olympus file stands up pretty well.
The D600 files are clearly cleaner and look "better" to me. (Nikon at ISO 100, Olympus at ISO 200 - each cameras base ISO)
The D600 file has significantly less DOF. All images were shot at f/8.
Though focal lengths are supposedly "equivalent" there is clearly a difference in the spatial relationships between the objects photographed.
There is far less difference between the files than you might imagine.
I'm not trying to make any point here other than my usual one about the differences in image quality between modern digital cameras being less pronounced that is often made out. I'm also not trying to say that either is better, worse or more useful for me. I use both cameras and will probably continue to do so, at least in the immediate future. Since I have different cameras and different formats, I do find it useful from time to time to see what the differences are between them, and I often find that assumptions I have before testing can be changed by what is actually the case.
If you want an in-depth ananlysis of noise etc. and high ISO performance then there are other sites that do this better than me. The Imaging resouce comparometer is a particularly excellent tool for checking one cameras files against another and I recommend it.
The camera(s) I choose to use at any given time is the result of many things. Sometimes well-considered but often impulsive as well. "I fancy using that one today" is often the reason, and there is nothing wrong with that. Different gear and combinations of gear can be a source of stimulation and I would go so far as to say inspiration. I remember both Snowdon and David Bailey talking about the "benefits" of using something that you aren't that familiar with. Sound crazy? Shouldn't we all learn everything thoroughly and know every page of the manual before we ever use anything? Well we could, but I've always been of the opinion that using the same gear in the same way in the same situations leads to a "staleness" that becomes all too obvious in the pictures we take. A bit of a challenge every now and then and the need to learn "on the job" has always struck me as a way of sharpening the senses and putting a little "edge" into what I'm doing.
You might think that this is probably not a good idea in a situation where you need to do a "good job", though I once did a wedding with a camera I'd bought the day before and managed to press a few buttons I shouldn't have and got lost in some incomprehensible menus. However turning it off and on again and hitting auto-everything solved the problem. Irresponsible? Unprofessional? Well maybe, but I'm glad to say my instincts haven't failed me yet and as photographers we do (fortunately) have that auto / programme fallback. (Better known as the "idiot button")
I do like a bit of "serendipity" from time to time and as I visit a lot of the same locations on many different occasions I do try and use a different camera and / or lens every time I revisit. I have unsuccesfully tried to discipline myself to one system and even one camera, but this always seems doomed to failure, as does my avowed intention of getting down the number of picture taking devices sitting on my camera shelf.
I'm in the fortunate position of being able to have a variety of cameras and lenses to choose from, though fortune has little to do with it. Its my job and being a sad photo geek I don't spend my money on anything else anyway, so its not really that much of an indulgence. Plus I would have thought that it was obvious by now that I love using this stuff, and the intriguing combinations that I'm able to put together. My protestant ethic guilt at this "excess" however is often difficult to reconcile and coming from a background where spending money on such decadent luxury items was not an option, I often find it difficult to justify what I do to the internal "sensible person" that lives somewhere inside me.
A recent article I read on the BBC website seemed to indicate that "creatives" are more liable to be "nutters" than other normal (therefore boring?) people. I may not have phrased that in the most PC way possible but you probably get my drift. Those of you with perception (i.e. all you barking mad artists out there) will have worked out that this is a long-winded explanation (excuse?) for the fact that I can go out shooting with a Nikon D800E and expensive zoom one day and an Olympus E-PM1 plus 17mm f/2.8 pancake the next for no other reason than "I fancy a change". However it doesn't seem to be doing me much harm. I mentioned a few days ago that I was set to sell 1000 images in a calendar month with one library. That was duly achieved earlier today and I took this screen shot just after it happened to commemorate this historic occurance.