Go for the RG-6, as say specifically says RG-59 will not work with DishPro. My guess is they advertise that existing wire works because most cable companies have been installing RG-6 for about a decade. Odd are that the house is pre-wired with RG-6.
If you can, get a nice tri-shield RG-6, and use quality compression fittings. Best deals on cables I've found is comercial electrical companies. Most have "city desks" where you can buy small (250-500ft) spools of coax, usually at 25-50% less than your local builder store. Greybar and Viking and two national companies I use. Just tell them you're a indie installer as they typically don't want to deal with the public. Many DBS installers are indie and have to buy their own cable and parts anyway so they are used to it.