I agree with Rorschach. You need to see what feels best in your hand.
I had the same dilemma as you for 3 weeks. I ended up reserving both at Jessops (Canon with 18-135mm kit lens and Nikon with 18-105mm kit lens).
I took them both home, unpacked them, and played with them for a few hours. I took a few pictures with each (not too many as i was conscious one would go back) under different lighting conditions. After those trials it was decision made; the Canon 550d with 18-135mm kit lens was the one for me.
Both are great cameras, and TBH the Nikon felt more solid and professional. However, the Canon 550d was not so heavy (but not overly light and cheap), more comfortable in the hand for longer periods and (IMO) had noticably better image quality straight from the camera.
The video function on the 550d was also much better than the D90, but to be honest I could not be less interested in video!
The menu system on the Canon for me was more intuitive and the new "Q" menu made it very easy to change settings.
I had previously owned a Nikon D40 and Canon 450D, so have no axe to grind in the Canon versus Nikon war; accepting that my more recent camera was the 450d.
All that said, at the end of the day, only you can decide, others on here will have completely differing opinions because we are all different.
What i did with Jessops (in buying both cameras and trying extensively) was a bit cheeky, but for me I pay a little extra to buy from Jessops for this cheeky little act