From: Konin, Poland, European Union
higher max speed, lower endurance and range, higher max alt and climb. That's it, everything else is identical.
Kates can carry torps to 7 hexes, so I would use D3A1 even if it was more vulnerable than D3A2, just for a chance to have some more meatshields for your heaviest hitters in 7 hex strikes. And 6 hex to 7 hex is a big increase in area covered (~35%).
"Only the Dead Have Seen the End of War"