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Getting a EM-5 and just using one lens seems to sort of defeat the point of the whole m4/3 system, which is all the great lens. 7-14mm, 12mm f2.0, 25mm 1.4, 45mm 1.8 100-300 zoom etc etc
Not to say that you need every and all lenses of course, but really the lenses selection is one of the strongest parts of the format.
Having had both the X100 and EM-5 though they are somewhat hard t compare.
The EM-5 works basically like a mini DSLR, good EVF, nice flip out touch LCD, amazing IBIS, fast AF, tons of customization etc
X100 is rather more like an old RF camera in its overall control layout and way of shooting. Its got a great lens, amazing IQ, and a cool hybrid OVF/EVF.
Its AF is much improved to date, but still not as fast as the EM-5. It does edge it out in terms of pure IQ, especially at high ISO, however the IBIS does allow the EM-5 to stay at lower ISO's longer (unless you need to stop motion)
Really though both cameras can take fantastic photos so IQ isn't really how I'd choose
X100 has a great built in flash and with its leaf shutter can sync up to 1/4000th, very useful for fill flash. EM-5 needs a clip on accessory and has a normal sync speed. May or may not be a factor
X100 handles a bit better to me as well, but it depends on the lens you put on the EM-5. 20mm 1.7 is pretty small, but I think the EM-5 really needs a grip so its more of a jacket pocket vs cargo pants pocket size
X100 can be had for $999 new
EM-5 is $999 for body only, and then add $300 or so depending on lens you want, 20mm 1.7 would be a nice choice. Also I think the grip is needed and that is $300 as well.
Now your looking at $1600 vs $999. Big difference in price
Also might want to consider something like the Sony NEX7 and a Sigma lens. Great IQ from that combo as well
Another even crazier option is the Sony RX100, which I replaced both my EM-5 and X100 with!
Works for me because I've got plenty of DSLR's for my serious photography work and I wanted something as compact and pocketable as I could find.
RX100 is perfect in that regard because it gives me IQ that an match m4/3 with kit lens but fits into any pocket.
Certainly wouldn't use it as my only camera, but if your looking at something as an alternative just for traveling light or day to day carry its tough to beat.
Its 28mm f1.8 on the wide end as well and really performs nicely as a general snapshot camera in that regard, even in lower light.