Between the two Canon cameras, I'd suggest the 550D because the flip out screen on the 600D seems unnecessary to me. If you're shooting food in a studio then you could shoot tethered to really nail the focus on your computer monitor rather than the tiny little display on the back of the camera.
But ultimately, the camera itself is the least important part of this equation. For food photography you need fresh ingredients, food styling skills (or a stylist), ability to control the light, and post production skills (like Photoshop).
If you're just blogging (as opposed to being in print) then you probably don't need to be so strict because the final image size will be smaller.
The Nikon will perform slightly better in low light... but for your work your goal should be to always shoot with lots of light at ISO 100 for minimal noise.


