Canon & Nikon are the two 'main' makers although Sony are producing some very good cameras now and I believe make the sensors for Nikon. Which way you jump is usually a matter of whatever suits you best, they all have differences, only real way to tell is to play with them all in a shop. Some people prefer the button layouts on one to the other, some the menu systems. Whichever you choose you're kind of locked into as lenses etc. aren't interchangeable but third party makers such as Sigma make the same lenses in both Canon & Nikon mounts.