HG785 versus Q235: the difference shows up in month 18, not week 1.
A Q235 body dents where the excavator bucket drops rock. Each dent is a stress riser, and after a year the floor starts to crack along the dent lines.
HG785 has roughly three times the yield strength, so a 5 mm HG785 floor outperforms a 6 mm Q235 floor at lower weight. On a tipper that also means more payload for the same gross weight.
The cost difference is about 8 % of the body price. On a unit that runs 200 km a day for five years, that is the cheapest 8 % you will spend.