The Tern looks very similar to the Teric... looks is personal taste, you do you...
Polymer is durable for sure but more then metal?... not sure, and doesn't really matter, you hopefully wont drive a tank over it. But IMHO a metal housing ages more noble... (eg
Shearwater Teric or Perdix?), plastic just looks bad after years of demanding usages. I use the Teric over 8 years/+400 dives, never had any issues and if someday, Shearwater support is legendary. Nice point is also that Shearwater keep their value pretty well, still could sell mine for almost half for what I paid.
In terms of DC function you're right, I may have jumped to conclusion here. Never dived a Garmin, I know many love it and for sure it's a capable DC, also the G1 (or any Descent) is the better every dive day carry. I hate that the Teric has no option to keep the watch on. However, I want a dive computer that just works for many years. Garmin released their MK1 in 2017 and heard they basically stopped supporting it last year.