The 105 satellite will not work in some of the northern and southern states in the US. The 121 works in the areas where 105 does not. Those that are using 105 will be using a new satellite that will be placed there later this year. Once the channels at 105 are placed onto the new satellite then the signal strength should go up and those areas in the US that cannot receive the signal very well at this time should be able to.
There are two types of SuperDishes, and two versions of each one. Type I is metal and uses 2" poles/masts and is lighter than the Type II which is plastic which uses a 2 3/8" pole/mast. One version is the 105 SuperDish used for the 105/110/119 orbital locations and the 121 SuperDish is used for the 110/119/121 orbital locations.
If you have locals in your area then you can get the SuperDish free including basic installation with credit card/checking account autopay or $24.99 at participating retailers with a one year commitment to AT60 or higher and your local package.