Agree with the other posters- its important to decide how large you may want to go with folders/dies.
This link takes you to a site that has a great compatibility chart of dies/machines.
Personal recommendation? Go as large as you have space/finances for. I bought a 'bug years ago, and got fed up with dies/folders I liked getting too large for it. With that redundancy in mind, I opted to go with a BS Pro- 12 1/2" wide opening, and able to take any die size on the market. I figured there is never going to be anything larger than this machine can handle I am likely to want to use. I still use my 'bug for small, quick stuff, but like that I can run more than one die/folder through my BS Pro at once, as long as they are the same type.
I also have a Silhouette, and in case anyone doesn't know, Silhouette store has all shapes half price until the end of March!