I have tried to compare the two systems with similarly featured cameras and reasonable lens options to show what the price difference is and image quality at that price.
If you are shooting fast moving subjects in AF, or you want to do remote TTL flash, or use large, heavy lenses, then the Nikon dSLR would be your best bet, but for everything else, a mirrorless camera systems will do as good, if not better, and should give sharper images edge-to-edge, better HD video with ability to do slo-motion and with a far more compact kit.
Furthermore, if you are financially challenged and don't mind having fun with legacy lenses, you can put almost any lens you can find on Ebay onto a mirrorless camera systems camera such as the G3 - just get an adapter. With Nikon, you are stuck using Nikon F mount lenses only, and even then, the Nikon D3100 will not autofocus certain lenses such as the Nikkor 50mm f/1.8D BUT you can use the newer, more expensive AF-S 50mm f/1.8G lens.
It will be very interesting indeed to see how the new Olympus 45mm f/1.8 lens compares in lens testing with the options for Nikon:
In the end, it will be the lenses which make the difference to your photography, and there is no point factoring in lenses you cannot afford. The ability to use cheap legacy lenses should not be under-estimated.
The new HD video optimised, fast CD-AF lenses for Micro Four Thirds make a compelling case for many people:
In addition to the above there are some very nice compact lens options for Micro Four Thirds including:
|Panasonic G3||Nikon D3100|
|price with 3x kit lens||$849 sharp||$779 not very sharp|
|price with twin kit lenses||$1099||$1049|
|10x kit lens||14-140mm OIS better image quality, HD video optimised||18-200mm VR soft at 135mm,lots of distortion, not HD optimised|
|wide aperture standard lens||$365 (Pan. 20mm f/1.7) sharper, tiny||$349 (35mm f/1.8) soft away from centre|
|wide aperture wide angle lens||$799 (Oly. 12mm f/2.0) awesome 24mm f/2.0||$759 (20mm f/2.8) very average 30mm f/2.8|
|wide aperture portrait lens||$399 (Oly. 45mm f/1.8) nice HD optimised 90mm||$220 (50mm f/1.8G) SWM for quiet AF 75mm only|
|macro lens||almost any legacy macro||Nikon F macro only|
|phase AF||none but CD-AF is at least as fast for slow moving subjects||11pt fast|
|HD video||1080i 60 fps stereo||1080 24fps mono (no slo-mo possible)|
|LCD screen||460K swivel touch||230K fixed|
|remote TTL flash||no (Olympus Pen models allow remote TTL flash)||yes|
|built-in IS||no (Olympus Pen models have IS)||no|
|ability to use legacy lenses||nearly any lens ever made||only Nikon F|