Aesthetic, construction material, sound dampening material. That's about it as far as differences go. The all metal body of the fortress does drop temps a degree or two.
If I were limited between the two, I'd base my decision on my concern of sound (though the raven is damn silent as well). The extra 60-70 dollars wouldn't be enough for me to justify an all aluminum body.
One last thing to consider is if you plan on keeping your pc on a rug, then stretch for the fortress. It uses a platform instead of feet to keep it raised for fan clearance.
http://www.nzxt.com/new/products/crafted_series/switch_810
Getin me one of these, soon as it starts showing up at retailers. Something to consider.