I have found The Dish numbers are accurate. However, there is going to be imprecision because if content doesn't have a lot of movement or objects, such as a talking heads show like Oprah or a movie like My Dinner with Andre, you can expect that content to take up less room on the HDD. However, if the content has more action and more detail such as a sporting event (X Games) or an action packed movie, then it may consume more space than the talking heads show on the disc. This is true for both SD and HD. However, this isn't going to make a big difference on your HDD capacity, as the variety of content you record will generally even things out in the end. This is how MPEG works and is not unique to Dish. The same is true for any DVR, Tivo included, as this is the effect when digitizing moving pictures. Dish does have a disclaimer regarding its maximum capacity for DVR's by stating that actual capacity depends on the content of the programming you record.
Having said all that, the 55 hours for the 722 is quite accurate. However, you will want to invest in an external HDD to plug into your HD DVR as you will quickly fill-up your HDD with 55 hours of content pretty quickly.