I would select Hilton if you go to smaller cities, and probably Starwood if you stay in big American cities.

Starwood is great if you or your employer can afford the Sheraton/Westin/etc level, and they have the incredible Lurker and no blackout dates. But Four Points and Sheraton Inn properties are often very very sad and tired compared to Courtyard or Hampton Inn. Many of the places where I travel either do not have a non-4Pts Starwood brand, or its at a premium of around 50%. I don't stay at Marriotts because I inevitably get hassled about using goverment rate when travelling as a contractor on government projects whose contracts specify that I qualify for and have to stay at/below government rate.

I really value consistency when I travel for work, and except for the ex-Red Lions that are now Doubletrees in the Pacific NW, I have been very satisfied with Hilton.