With your flight patterns I would certainly recommend HP. I used to fly HP exclusively for some time, but their program didn't work for me very well (as posters above mentioned their Intl redemption is almost non-existant), but for a domestic flyer with 6-9 flights a year HP program will certainly benefit you better.
United upgrades are hard to come by unless you have a lot of miles and hold a mid tier status with them.
HP upgardes are easy to get, even for a Silver.
Also, you ever considered AS MileagePlan? Its VERY good, better than either UA or HP, and they fly to all of your destinations. If I were you I would certainly give AS a better look.
Happy Travels!
[This message has been edited by asnovici (edited 07-09-2003).]
[This message has been edited by asnovici (edited 07-09-2003).]