I think the answer for you also depends on what kinds of hotels you want to stay in. I know a lot of people are fans of Starwood and Hyatt, but I find they're often a little pricy for my budget when I'm paying for myself. Hilton and Marriott have a pretty good range of hotels (from moderate to luxury) with pretty good coverage in that range in a lot of the places I visit. So I can afford to pay for a mid-range hotel with my own $, and use points to splurge for something top of the line. In some markets, however, Starwood does offer a somewhat wider range of hotels (Paris and the Middle East come to mind), so it's worth a look, too.

While Starwood has lower point thresholds for rooms, I find it a lot harder to accumulate Starwood points than Hilton or Marriott points without hotel stays. That's because I can get Hilton and Marriott points by clicking through their portals to internet retailers, and I can get Hilton points for eating at certain restaurants, in addition to the points I get for expenditures on the card.

By the way, I have the Hilton and Starwood Amexes, and the Marriott Visa. All but two nights in our 2.5 week trip to Egypt this fall were paid for with points, and we stayed at great hotels in each of the three chains.