I've heard people refer to the fact that HL plastic is a bit more brittle than Tamiya - not in a major way, but it makes a difference when you are trying to cut or shape it. (ABS vs Styrene???)
Mato's plastic was suppose to be closer to Tamiya's.

HL's are built more like toy store tanks, and Tamiya are hobby shop / RC tanks. HL uses self tapping screws into plastic where Tamiya tends to use machine screws, nuts, and bolts - especially in parts that are high stress or frequently assembled / dis-assembled. Tamiya also uses stamped metal and aluminum - which they should considering the price difference.