First DSLR: Nikon D5100 or Canon 650D?

Go into a shop and pick them both up. Which ever feels more comfortable & natural in you hands is the one that you will want to use the most, so pick that. Once you have become used to handling the selected camera and have explored all of its capabilities you'll find that many (maybe all) of the technical differences fade into insignificance. You can take superb photos with both of these cameras.