I'll chime in here on Starwood too. Since our favorite type of vacation is a Biz class seat to Europe, and staying in nice hotels there, we like Starwood the best.

If we need to top up an airline account, there is the transferability discussed above, plus the bonus miles on transfer.

But it is the lack of capacity controls that we enjoy the most. If a Starwood hotel has a standard room available to purchase, you can use points for it. That has enabled us to get rooms when other chains did not have any award rooms available (or only had them at premium rates, such as Marriott "Stay Anytime" -- by using 50% more points).

I know that some have complained that this Starwood policy is not always followed (although some complaints seem to be special events, such as Superbowl or Final 4, which appear to be excluded), but in our experience, we have had good success using SPG in Europe.

I've gotten tired of trying to use Hilton and Marriott points during low season in places, and have them still tell me that they have no awards available.