Here is an excerpt from a detailed (shortcut below) comparison between the D5100 and the Rebel T3i
"[FONT=Geneva, Arial, Helvetica, san-serif]For those however considering to purchase a camera with the standard kit lens, we would recommend the Nikon D5100 kit with the AF-S DX 18-55mm VR zoom as offering the best solution, since the results delivered by the Canon EF-S 18-55mm IS II zoom lens when matched with a high resolution camera like the Rebel T3i display a fair amount of weakness in comparison. [/FONT]
[FONT=Geneva, Arial, Helvetica, san-serif]In general, the Nikon D5100 with the standard Nikkor AF-S DX 18-55mm F/3.5-5.6G ED VR zoom offered better performance compared to the Canon Rebel T3i with the EF-S 18-55mm F/3.5-.5.6 IS II zoom kit lens. This is not only as a result of better optical lens design in the Nikon AF-S 18-55mm VR, but also because the lens is a more suitable match for the D5100's sensor. Another factor is that the D5100 incorporates technologies such as automatic 'Chromatic Aberration Correction' which are specifically designed to enhance lens performance.[/FONT]
[FONT=Geneva, Arial, Helvetica, san-serif]Irrespective of the lens used, the Nikon D5100 offers better sensor output compared to the Canon Rebel T3i, especially in terms of minimizing digital noise when shooting at high ISO settings. The Nikon D5100 sensor offers improved colour depth, dynamic range and high ISO capability versus the Canon Rebel T3i / EOS 600D, as supported by our own and DxOMark Labs sensor comparison tests.[/FONT]
[FONT=Geneva, Arial, Helvetica, san-serif]If being able to shoot digital SLR type HD movies is an important requirement, then the Canon Rebel T3i / 600D offers advantages over the Nikon D5100 by providing manual exposure and added sound control features desired by more advanced users. "[/FONT]
http://www.digitalreview.ca/content/Canon-Rebel-T3i-EOS-600D-Compared-to-Nikon-D5100.shtml