I know if you have an OPF (overseas pakistan card/membership) you get like 50% or so discounts at the Mariott in Islamabad which is BIG. Mariott is rather expensive but the location is great and its mostly foreigners, not paki foreigners. Serena is new but for some reason still not as popular as Mariott. Holiday Inn is decent but its a bad area of town compared to the rest. There is envoy continental in Blue area and crown plaza, I hear their good, and they look decent inside, except captured from the late 70's or early 80's!! Dont settle for their price on the brochure, tell them how many days you plan on staying and argue for a better price and make you you argue the price INCLUSIVE of taxes because a hotel room that cost 3000 rupees a night will end up costing 4000 or something without taxes, their really sneaky about that.
Unless you like cockroaches and low standards in a Paindu atmosphere, I suggest staying away from Guesthouses.
And last, if you like to shuttle between Pindi and Islamabad quite often, there are quite a few hotels near Islamabad club which is in the middle of Islamabad and Rawalpindi and their quite decent, Margalla Motel, Best Western, etc. But heck like you said, budget is no problem, go for Mariott!
well in rawalpindi u can stay in shalimar, hotel akbars international, pc their rooms and food are realy good and in islamabad u can stay in dreamlands motel its service and food is good and also margalla and if u want luxurious rooms best location and expensive food which os not always good u could preffer marriot in isl.
if u want cheap and yet somewhat reasonable place, go to New Islamabad Hotel in Aabpaara market....
i stayed there for a night, in a double bed room with AC (u have to ask for this luxury) for just Rs 1800....
i dont think u can get much cheaper than this....