While I don't have any firsthand experience with the Huskees or Simplicity, I can say that the X300 is a wonderful machine to use and is hard to find fault with.
They are extremely smooth to operate and the controls are effortless to use. If you purchase new, JD will offer a "Promise Buy Back" if you aren't happy with it within the first couple weeks of use. They come with a nice warrantee and purchasing from a dealer has its avantages as well versus buying from a box store. A dealer will assist you with any warranty claims and they may also offer other incentives as well if you tell them you've been considering purchasing from a retail outlet instead.
Have you by chance driven either of these machines? If not, I would highly recommend it as by doing so you'll get an idea pretty quick what you like and don't like about each.