The Starwood Preferred Guest Program has over 30 airline partners. This is more than the Membership Rewards, Diners Club, and Chase Ultimate Rewards program. The screen shot below shows the list of airline partners.
Notice that for most airline partners, the transfer ratio is one for one. Furthermore, if you transfer 20,000 StarPoints to your FF miles, you will get a 5,000 bonus miles from Starwood. You are allowed to transfer up to 79,999 StarPoints within a 24 hour period. That means including your bonus, you will move 94,000 Points (79,999 + 15,000 Bonus Points). Because of the bonus feature, you should be transferring your points in 20,000 StarPoints increments.
Another feature that Starwood has added to their program is the ability to book flights directly with StarPoints. In fact, the number of points required versus the value of the ticket is listed on their site and below is a screen shot.
The value of StarPoints if you choose to redeem your points directly this way with flights is slightly more than 1 cent. While this is more than the typical credit card reward programs, most savvy frequent fliers will get about 3 or 4 cents or point by either using it for a higher category hotel or by transferring to the right frequent flier program and getting either great international business class or first class tickets with award points.
Perhaps the only weakness in SPG offerings is that the transfer ratio for United is 2:1 and SouthWest is not a partner with them. If you fly those airlines and are looking for an alternative or an addition to their affinity cards, then you have to go with Chase because their Ultimate Rewards program has United and SouthWest as their partners. The Chase Sapphire PreferredSm Card would be the card to get then.