Olympus vs. Nikon...

Nikon and Canon jointly have most of the market share. That has several real effects. You're more likely to have a friend with one whose equipment you can borrow. You're more likely to find used gear. Olympus, on the other hand, tosses in a variety of features that Nikon doesn't have (anti-shake for any lens, LiveView, etc.).

Keep in mind that your older Olympus lenses and whatnot are incompatible with the new Olympus bodies. The name on the camera is "Olympus", but it really should be "4/3 system", since that's the set of compatible lenses that you can buy. That notably includes several clever and pricey Leica-branded lenses.

Ultimately, you should go hold the cameras in your hands and play with them in person. Like Patrick, you may decide that the feel of the thing as you hold it makes a difference.