You are correct in your comments about the size of the unit. The Model C is the only one that you can use to get 120" wide. The biggest differences between the Model C and the Model B is just that, SIZE. However, having said that there is more that just the available image sizes that are larger on the Model C. The case and roller are also bigger. The case is formed a bit differently than the Model B and it is made out of a thicker gauge of steel. Also the roller is a 3" roller with a ball bearing spring mechanism rather than the 1-7/8" on the Model B. The fabric however, is the same type regardless of the unit. You just obviously get more when the screen size is larger.
One word of caution however, I am a bit concerned about your wanting to use a 120" wide screen. If you use that wide of a screen you will have to have a projector that will output at least 1,500 ANSI lumens and the room must be dark. If any of those factors are not met you will need to reduce the screen size. On the other hand as long as the projector is bright enough, you will have no problems.
I hope this helps.