Is it some deal where you get it for free?
I was going to say (not meaning to flame) that both suck. It seems that 90% of the movies are the same, month after month after month.
If you're considering paying for one or the other, add netflix to your poll. Probably cheaper, and you'll have a better selection. Before I moved, I had all of the pay channels... HBO, Starz, Cinemax, TMC, etc. Over a span of 2 months, I tried to record as many movies as I'd like to watch - I have a dvd recorder. Every day, I'd look and see if there were any movies that I wanted to record to watch at a later time... every night before going to sleep, I'd look through the listings. After 2 months, and 30 movies (most of which are still frequently on), I realized that it would have been far cheaper to just get netflix.
6 months later, we had all the channels available again to us (moved and switched to directv; extra channel deal at a steep discount for a few months). Most of the same movies were still playing. I'd go through the listings, quite annoyed, looking at when the movies originally came out - 1984, 1982, 1990, 2001, 1988,... Very few were new releases.
If you have to choose between the two though, I'd go with HBO.