I'm assuming 21oz isn't considered very thick? That Green Ulster DB looks lovely but I'm trying to figure out how warm/cold I'll be in it wearing a sportcoat or sweater. Chicago winters can get damp and cold but thankfully I drive to work...
Also I don't own any Eidos jackets but I'm typically a 38R in all my suits/jackets. I assume I'd take the same?
Yes, 21 oz is pretty average weight for overcoating. There are overcoating fabrics around 1000 grs in the swatch books.
The DBs will definitely be warmer than the SB (more coverage of course, but also longer)