I'll be building new screen/storm doors for my house this spring as well. The front will be white oak - great outdoor wood and matches my original entry door / sidelight assembly. The back door will be either cypress or southern yellow pine - something pitchy, known for it's outdoor stability, and locally available.
In my region (St. Louis area), Black Locust is the gold standard for exterior stability and rot resistance. Honey locust and mulberry are rot resistant as well, but it's my experience that they tend to move around a lot when exposed to the weather. The white oak is great - it's what whiskey barrels are made from - stable and fairly non absorbent.
I would not use poplar. It's not good in the weather. Maintenance will be higher.
Paul Hubbman