The Balmar Smart Gauge and the Victron BMV meter come at their answers in totally different ways.
The Balmar Smart Gauge measures the impedance of the battery and compares the result to a table stored in memory and to previous readings from the same battery to come up with an instantaneous estimate of battery state of charge. It doesn’t measure current flow in or out of the battery and cant tell you what is going on at the moment, it just tells you the state of charge and the voltage. Tests have shown the answer it comes up with to be reasonably accurate, but nevertheless there are some limitations to this method. It is not suitable for Lithium Ion batteries.
The Victron meters tell you what is going on now, ie how many amps are coming in or out of the battery. They calculate battery capacity by counting everything that goes in or out, figuring out over time where the full point is, and then counting down as you draw power. They assume an efficiency rating for the battery at the start but over time they calculate what the actual efficiency is.
With a water tank you can know exactly how much water is in it but with a battery the answer is not as precise. How much you get out depends on how fast you take it out and what the temperature is. Both meters have to deal with this and the Victron deals it by periodically resetting itself every time it gets discharged at least 20% and then charged back up to full. On an installation where that happens on a regular basis the Victron does very well. On installations where the batteries never or rarely get to full the meter never gets reset and so over time it can become more and more inaccurate.
So which meter is best? There is no one answer. On a starting battery that doesn’t get discharged much the Smart Gauge will win every time because the conditions the Victron will need to reset itself never happen. Same thing for a boat that lives on a mooring and rarely gets to shore power so the batteries don’t often get fully charged. On an installation where the batteries get regularly discharged and charged back to full I think the added Amps in or out information provided by the Victron gives a better overall picture.