Olympus OM-D E-M5 or Nikon D5100 ? Need your advise

Hello!

I want to buy my first serious camera after years of using compacts. The budget is around 1500$. I need kit zoom and one standard prime lens at the beginning. Can't choose between these two sets:

1. Olympus OM-D E-M5 12-50mm lens 1300$ + Sigma 30mm F2.8 EX DN- Micro 4/3 200$. + 250$ delivery from USA to Kazakhstan. Total - 1750$.

2. Nikon D5100 Nikkor 18-105mm f/3.5-5.6 AF-S DX VR 1100$ + Nikkor 35mm f/1.8 G AF-S DX 280$. Prices from local stores. Total - 1380$.

Never seen Olympus OM-D E-M5, just read the reviews here. Olympus pros for me are light weight, dimensions, and its weather sealed. So my questions are:
1. Is it worth extra 400$ ?
2. How is Olympus 12-50mm lens compared to Nikkor 18-105mm ?

3. What about sensor quality, low light performance, speed, ease of use of these cameras ?
Thank you!!