HMI is a much better designed door than a therma tru especially from an engineering/ design perspective. Much more solid.

Fiberglass and steel are both good options. Personally I prefer FG, but again, both will serve you well for a long time as long as you get a good product. Regarding the storm, you'll want to vent that space during the summer whether it is steel or FG to reduce excess heat buildup. Both door materials use plastic window surrounds, and that is typically the first item to show damage from heat.
Secondly -- and more importantly, if you are in the DC area, you need to contact Windows on Washington. Phenomenal company.

Agree on windows on washington,,
HMI have a built in ventilation system in their storm doors in order to prevent any heat build up between the enry door and the storm . Very cool feature.