I currently have Showtime-HD and very little of the programming is HD. Tudors is HD, but recently filmed Penn&Teller episodes are not HD. Other movies that I tried were sometimes not in HD. I would not recommend that choice unless you were looking for something specifically on Showtime.
I'm going to subscribe to HBO-HD on Friday, because they will be starting "John from Cincinnati" on June 10th - the new TV show by David Milch (Deadwood, NYPD Blue, Hill Street Blues - I thought Deadwood was easily the best show of the past 5 years). And, on Friday, HBO-HD will show all six Star Wars movies in HD (which won't be on HD disk for a number of years, evidently).
PS There are probably more HD movies on the "Dish HD" package than anywhere else. If you like Horror or Martial Arts or Art-House Films or International Films, there's no contest, as each of those genres has its own HD movie channel. And there's also HD-Net Movies, which shows more movies in HD than any of the Premium channels, although admittedly the titles are not as recent.