We have both a Thetford and a cheaper version (the type most places cell under their in house brand names) The Thetford is better made, out of a shinier/easier to clean plastic, but the cheaper one works just fine as well.
The main difference is when emptying, the Thetford has a tube to tip out of when emptying, this makes it a bit easier to control when tipping it out and therefore greatly reduces the chances of splashback, especially if you are emptying into a toilet. The trade-off though is as it restricts the flow a bit, you sometimes might need a number of goes rinsing out with water to get any fresh clumps of toilet paper to actually come out, whereas the cheaper generic ones have a much hole in the side and no restrictions so everything comes out much more easily.
Both work just fine, I think the Thetford one is better, but whether it is worth the extra $$$ is probably the question, and that will change from person to person.