Notifications and battery life for me, plus I really like the negative display of the GBD200. It's awesome seeing from a quick glance if a message is worth taking out my phone from my pocket or not, and not having to worry about charging it every other day.
Gadgetbridge on Android supports it well and I can choose which app will send a notification to it to cut down the noise.
Nice. A smart watch is on my radar, I just hate how invasive most of them are. Gadget Bridge isn’t an option for me currently, but that may change soon.
I've heard the EL backlighting makes the LCD a bit less legible. I don't know if that is true but I do appreciate the legibility of the 5610U, especially when wearing it on the inside of the wrist like I do.