I agree it helps to know which direction you want to go, and also what your needs are. If you can figure that out first, and know what route you want to go, picking the battery becomes easy.
DC-DC provides more isolation and can charge and maintain the aux battery completely independent of the starter. Alternator and starter system voltage is completely isolated from this system on the AUX side.
The typical combiner, automatic charging relay, and similar device will combine or disconnect the two batteries in parallel. Generally, they are capable of moving a lot more current than a DC-DC. Sized correctly, could even be used to combine the aux for large loads like winching. Alternator is also in parallel when combined. Both sides share the same system voltage when combined.
Both system have pros and cons.
I generally use a DC-DC when working with custom voltages, like a 24v system in a 12v truck, or when needing to charge physically smaller SLA batteries, or better maintain AGM’s if the native voltage of the truck doesn’t fully support the top end charge voltage.
Here is a thread I made a while back that might help your research.
Blue Sea ACR - Automatic Charging Relays