When would a CM820-series router suit better than a CM685-series model?

That depends on the exact model and the surrounding job. In broad terms, a CM820-series router is often part of a more industrial-style deployment conversation, especially where failover, stronger local interfaces or unattended service matter. A CM685-series model may suit lighter-duty or more straightforward jobs where the essential connectivity need is clear and the environment is less demanding.

The useful comparison is therefore not series versus series in isolation but series versus application. Comset can help narrow that down by looking at the site conditions, data load, interfaces and support expectations.