5N takes Full-HD 1080x24/30/60p videos vs. 720p on the C3 and features a magnesium alloy body vs plastic on the C3. Oh, and the 3 don't has the infrared port. And the 5N has a built-in touchscreen, where the C3 has the same display without touch.
However, image quality is the same.
The 5n also gives you the ability to use the electronic viewfinder and to set a base ISO of 100 (rather than 200).
I'm sending my nex-5n back as its got that clicking sound when replaying video. Don't know if they just repair it or replace it though.