Lee is British and the gel is color dipped, while Rosco is American and the dye is baked in, which is why Rosco gels last longer. The color differences are usually attributed to the different tastes between the US and England. Rosco makes a Euro color collection, which is closer to Lee and also dipped. I prefer Lee for CTB and CTO but Rocso for theatrical colors. As mentioned earlier, its best not to mix the Rosco and Lee versions of the CTB, but they are only off by a little. The CTO is also a bit different. They both work just fine however.
author, new book "Lighting for Cinematography"