I have the dishtruhd+ small box. I feel the Dish HD+ is same box but slightly bigger, they just changed the name to look in sync with SD+ name & for the sake of some changes they have changed the position of USB port, that's all I could make out from outside.
There might be small or big changes to the hardware & coz of that quality of viewing could be good as mentioned by Abhi Bro. Also keeping in mind for the future software to run on this box right from the release date, this is just my guess.