The subject vehicle (original post)is a 1991 model. From what I could see of the construction of the old evaporator, I believe the leaking problem is related to Behr's use of copper tubing which had to be mechanically sealed to the aluminum core. It looks like the dissimilar metal interface caused corrosion and eventual leakage of the mechanical seal. The replacement part is all aluminum with welded construction.
As far as using aluminum vs. copper for the overall construction is concerned, most manufacturers today use all aluminum evaporators for R134a systems. One does have to be careful, though, not to introduce moisture into the system during charging as the moisture forms an acid that will oxidize the aluminum core from the inside out.
I learned these things from reading everything I could find about R134a conversions, including a manufacturer's conversion instructions (Sanden), before attempting the job.