I
believe there are two separate units because the first (D250) was designed to charge just the starter battery and the later (Smart Pass) was designed just to charge a house battery. So they designed them to be able to work together if you wanted to do both rather than invent a whole new third device. That conclusion comes from my brain though, not from anyone at the company. I haven't talked to anyone at CTEK.
At the moment, I am debating between battery monitors. As I see it, there are three options. CTEK has one that connects to your phone via bluetooth. Of course, what if my phone dies, right?
http://www.ctek.com/cy/en/chargers/CTX BATTERY SENSE
Also, AIMS has one that is designed to work with their inverter/charger that I plan on using. I like this one because I it also gives you control of the inverter from it's screen as well as working as a battery monitor.
http://www.aimscorp.net/LCD-Remote-Panel.html
The last option would be a completely different third party monitor, say something like the TriMetric, Blue Sky, or Magnum monitors. In my drawing, I have the AIMS REMOTELF monitor currently. I haven't heard any pros or cons of this device, but I do trust AIMS products. Thoughts?