I am Diamond with HHonors and Platinum with Starwood and, if I had to choose only one, I think I would go for HHonors.
There are things I like about Starwood properties, but HHonors almost always has some type of facility in any domestic location and the international sites are great--always offering wonderful breakfasts and lounges.
Obviously, if your schedule provides enough stays to qualify for two, you should pick Starwood and Hilton, or just Marriott. But, if you are hitting only about 30 stays a year, I would suggest HHonors as the best overall choice.