Button Layout on the D5100 is not a problem since it hardly doesnt have any extra buttons. Your friend probably read a review on a D90 or D7000 or something. But still I think the extra buttons on a D90/D7000 are on a better place then on a canon. With the canon its squeezed away on top of the camera. I dont see how that is ergonomic/anatomicly. What finger do you use to operate those buttons on a canon while you still hold the camera with both hands?. Well none because its impossible.
With Nikon D90/D7000 you can operate most buttons while still holding the camera body with two hands. Just use a thumb or index finger. I am not sure if you have to use the scroll wheel to change iso seetings on a canon but that would be impossible to to do at the same time.
Also the scrollwheel that you use to change shutterspeed or iso settings is not very convenient on a canon.
But thats just my opinion.
My advise is go to the store and try to hold the cameras. If you think you want to use fully manual mode a lot then the D5100 isnt to good a choice probably since it only has one scroll wheel and no extra buttons, but the Canon 550D has the same problem there.
If you just use semi automatic shutter priority or aperture priority modes (camera will figure out the rest) then a D5100 would be nice.
For ergonomic reasons I would choose a Nikon. If you want a full manual mode camera a Nikon D90 or a D7000 would be a better choice.
Here a picture of the Nikon D90 so you can compare scrollbar and button locations: (d90/D7000 have a horizontal scrollwheel near the shutter at the front as well).
Just go hold both cameras and see what feels right for you. I probabaly would advise a D90 over a D5100 because of ergonomics in full manual mode. But then again if you want to use it for movies then D5100 is the better choice.
The Nikon D90 is absolute bargain in regards of (semi) professional hardware buttons and extra LCD display. So operationwise it would be far superior to both a Canon 550d and a nikon D5100.