Re: the Platinum card. I cannot imagine spending $450 for 50,000 miles from any airline, especially Delta Sky Pesos, even with the tranfer bonuses. I refuse to list it on my website, even though it would earn me mega commmissions from anyone using my link to apply for it.
There is a perfectly good 25,000 point bonus for the Premier Rewards Gold Card from American Express. The annual fee is waived for the first year. Points transfer to many airlines 1:1, including Delta, Continental (ends September 30, 2011), Frontier, Hawaiian, and Air Canada. Delta often offers mega tranfer bonuses. You can read about this card on my website below in the Credit Cards > Annual Fee Cards >American Express section near the bottom of the page. You can read about the bonuses for tranfers to various airlines on my Registration and Other Bonuses page.
After doing the required spend on that card, I would do further spends on the Starwood card if you want to transfer points to any airline except Continental or United. 20,000 points get you 25,000 miles. Sometimes there are bonuses for tranfers. I got a good one to U.S. Airways about a year ago. There is a regular 10,000 point bonus for getting the card. Currently you can get another 15,000 points for spending $15,000 in 6 months. There are both personal and business cards. I alternate applying for them every slightly less than 2 years (putting a charge on the one I have a month before the fee is due), to keep my points alive. Both cards waive the annual fee for the first year. You can read about these cards on my website below on the Credit Cards > Free Cards page.
There is no reason you can't get both the Premier Gold Card and the Starwood card from American Express. Then take a breath and apply for the Delta Amex card. Amex seems to have a limit of three cards at any one time.