Which airline are you going to be flying on most often? That would be the first question. Don't get an affinity card for an airline that you're not going to have many opportunities to travel on.

You should also take into consideration what you ultimately want to do with your miles. For example, if you want to go to Hawaii or the Caribbean you may want to go with Delta as the award levels are lower. But don't choose them if you want to go to Australia because the Skyteam award is high and the flights are not direct.

Another point: If you are considering the Delta Amex, the platinum card offers you a bonus. If you charge over $25K/year (and you will already be a third of the way there with your first purchase) you get 10K bonus miles.

pgary's site is a good resource for you when making this decision.