It is most likely which setting you are prefer. Final Blitzkrieg does have all the new features and is kind of good, but I don´t own it yet. Final Blitzkrieg ist about the late war in the west, approximately after Market Garden, so it is more a stand alone expansion to Battle for Normandy with all its features.
The fight about Aachen, Battle of Hürtgenwald, Battle of the Bulge. It does not covers a lot of the war, but the quality is very good, even if the game plays very bulky.
Which version you want to buy is most likely depending on the setting you would prefer.
There is an update for 10$ for the Battle for Normandy update to 3.0 version, so on the technical side Battle for Normandy is on the same level as Final Blitzkrieg and it is just a decision of the setting.
I own Shock Force with all modules (unfortunately this game is a bit out of date, some recent features are missing and the graphic in the newer games are better.) and Battle for Normandy with Commonwealth and 3.0 update. It is important to be up to date for multiplayer, which is really awesome in Combat Mission. Because of course you will need to have the same version as your opponent.
I also can play with people who don´t have Commonwealth forces, if I leave this units out, but not with people below the 3.0 update, version number musst be the same.
I can not recommend the Commonwealth forces, unless you are a real fan of the British forces, but they play a bit strange with their units.
I will buy Final Blitzkrieg and Red Thunder in the next time. I am not sure about the Black Sea part, which is a modern war in the Ukraine, developed before the shit was hitting the fan there... same is for Shock Force which plays in Syria... they really know about predicting the future.
However, if you want to buy Red Thunder, Final Blitzkrieg, Battle for Normandy or even Black Sea, there is not much of a difference, just pick your favorite setting,
I would love a mid 80s cold war setting, but this game is not in development yet.