I agree with you on Showtime, mainly because of the # of HD channels and HD VOD. We like a few of the Showtime original programming series, for example Dexter. So I wait until its well into the season or even the end and then download the whole season and watch them during the month.
I'd have to go with Showtime. It has the most HD channels (10), compared to 8 with Starz, 4 with HBO, and 2 with Cinemax. Also everything on Showtime is in OAR. Also all Showtime package channels have VOD (as does Starz), HBO and Cinemax don't.
Then if I didn't get Showtime for free already, I drop it after a month. No penalty and I watch the whole season for $12.99 or less. And my wife does like to watch old movies and the her favorites more then one time. So she has Showtime movies to choose from during that month. Works out fine for us.