I am not sure about the specific components between the two, but found this on Trek's site. Seems the level of aluminum is Red > Black > White.
http://www.trekbikes.com/us/en/compa...lpha_aluminum/
I bought my son and myself Trek 4300's (minus disc brakes) and have been MORE than satisfied with them.
Just my $.02
From what I was told when shopping around, Hydro forming is a process of shaping the aluminum resulting in less stress (from manufacturing) than straight bending/shaping. Using a liquid in the tube while bending and forming allows for constant pressure... or something like that. I'm not an engineer, just taking the info 3 LBS told me.