The D5100 is a good bang for the buck right now when purchased on clearance, and if you want to save money, size, and weight, you can get a Raynox DCR-150 or 250 instead of a dedicated macro lens and still get pretty good quality... 40mm is too short a focal length for many types of macro imho, anyway. 18-105mm is awesome for the price even if it lacks a metal mount.. that sucker is sharp. I owned one before.
E-PL5 is also very good but you won't get a viewfinder which may be a serious drawback in some cases (bright light/glare on the LCD, trying to photograph a fast-moving object). E-PL5 autofocus is great for single shot, not so great for tracking moving objects, but the D5100 doesn't do that much better, either... for that you would need a high-end DSLR or a Nikon 1, possibly NEX-5R/6 system though I haven't heard much about the PDAF in those systems yet. The M43 lenses are very small and light, which you will appreciate when lugging them around on vacation, etc. The 40-150mm is the same as the 40-150mm R but the 14-42mm II is definitely better than the older 14-42mm (it fixes a mid-focal length vibration problem, for instance), and it's a very sharp kit lens worthy of being kept around. I've owned both the 14-42mm II and 40-150mm before.
If I had to pick one or the other, I'd go Nikon D5100 simply because of the viewfinder which makes it generally useful; however, if I could pick another camera setup, I might go for a Nikon V1 or Panasonic G3 because those have viewfinders, and the V1 has crazy good autofocus on par with some of the higher-end DSLRs including tracking of objects and the lens selection, while small, is pretty decent; whereas the G3 allows you access to small and light M43 lenses.
Basically it all depends on how much you value AF and tracking; size/weight (of both body and lenses); cost; viewfinder; and lens selection.
If you don't mind not being able to switch lenses out, consider also the Sony RX100 which gives 28-100mm (effective) focal length. It can't track moving objects like DSLRs or high-end mirrorless, and you can't get as much subject isolation as you can with larger sensors or faster glass; and it lacks a viewfinder (but has a "sunny weather" mode that makes the LCD more legible than you'd expect). ...but it gives decent macro for what it is, and it's semi-pocketable and thus the most portable option among 1"-and-larger-sensor-size cameras.
I've owned and used a lot of mirrorless and DSLR cameras. Ultimately I sold them off and got a Sony RX100 while waiting for prices to fall to something more reasonable... there is no reason why mirrorless cameras should cost as much as DSLRs, which have more parts and which are more expensive to make.
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