inferior/weaker rims vs superior/stronger rims with an underweight cyclist
You get what you pay for. The DM18 is a cheap rim with good enough reviews rating it good value for money. The DH19 is a better rim - only you know if you can or want to stretch your budget, however "Quality is remembered long after price forgotten" comes to mind. Are you prepared to regret not spending that extra money every time you go for a ride?     

Provided the wheel is built well and maintained, the cheaper rim will stay true (its the spokes that keep the rim true, not the rim). The DH19 has eyelets for the spokes - meaning the rim will handle more abuse and last longer (without eyelets the spoke holes tend to fatigue and crack). However, as you are only 50kg, I expect the cheaper rim will easily handle the riding you are suggesting. If you were 150kg, I would steer you to the DH19.