I prefer the DOS because of the integration with the whole Apex system. It just makes it easy to see the actual dosage from the main fusion page. Easy to set up, easy to add logic to when or when not to dose, easy to change the dosing amount.
I actually have the Litermeter 3 with the extra pumps and they have been trouble free for 10 years, I moved them to my QT tank and got the DOS due to the ease of use and integration with Apex. Also had a couple Bubble Magnus, they both failed after a few years of service.
As far as longevity, I've only had mine for 4 months, but I assume they will last a long time. Most Electric motors last a long time unless they get over loaded, I think the major advantage of the stepper motor is its accuracy. I also like the fact that the pump heads and tubing are a single replacement part. Makes it easy to service.