I use a Nikon d2x, Nikon d3x, and a Nikon D300s. All three are great cameras, but are definitely not for beginners. Casually, I probably use the D300s more often. The cameras you mentioned (Canon 600d and Nikon d3100) are good for starters, as are the lower Nikon series, such as d60, d70, d70s, etc. Nikons are a bit more difficult to use than Canons, but they are both great companies. I prefer Nikons, though.