Which camera would be better for taking high quality large photos?

I need to take photos of approximatly A3 size at 300 dpi, would either of these cameras do this, and which one would be better for large high quality photos (one has a larger sensor size but the other one has more megapixels):
cannon EOS550 - 18 MP, 22.3 x14.9 sensor size, ISO sensitivity 100-6400, max shutter speed 1/4000 sec
or
nikon d3100 - 14 MP, 23.1 x 15.4 mm sensor size, ISO sensitivity 100-3200, max shutter speed 1/4000 sec

14mp is already enough

Either is fine, Make sure you shoot at the lowest permissible ISO, governed by your light source.
The lower the ISO the less apparent digital noise that will show up as you make bigger enlargements

The 14MP is good enough to print at that size and close tot hat resolution.

I'm going to use the T1i (15MP) as an example cause that's what I have.

A3 is 16.5 x 11.7 in
To print at 300dpi, your image would need to be 4950 X 3510

An image from a Rebel T1i (15MP) is 4752 X 3168 or 15.8 X 10.56 at 300DPI.

You'll need to print really 288dpi (or a little less) it won't really be noticeable.

You're fine with either camera. I have a shot from an 8mp Canon 20D on display at an aquarium blown up to 3x5 feet. Yes, pixels were interpolated by the printer. But at the expected viewing distance it is not an issue. The technical quality and aesthetic features of the image are far more important than megapixels.